My A99 versus D750 review, from another message board and adapted:
Let me start out where the 2 cameras are about equal. They both feel good in the hand, not too heavy. They both have nicely sized viewfinders. At base ISO, they both offer an amazing amount of dynamic range. And they both produce very similar IQ up to around ISO 1600, not surprising since the Nikon is using a Sony sensor.
Despite being 2 years older than the D750, the A99 has many strengths over the D750. I'm not going to list every tiny item, but I want to address the big items. The D750 has a pretty nice build quality, but the A99 has an even better build quality. The D750 battery door feels like it is going to snap off every time I remove the battery. The whole camera has a plasticky feel, where the A99 felt like a serious machine.
In terms of specs, the A99 has a couple of advantages. The A99 does have the faster maximum shutter speed, at 1/8000 vs 1/4000 and higher flash sync speed at 1/250 instead of 1/200. In a couple weeks of shooting, this hasn't matter to me at all. But it could occasionally come into play for some people.
But there are a couple other features, easily dismissed, that I will miss. Every vacation I ever took, I used the in-camera panorama at least a couple of times. That's missing. And the A99 has much more advanced in-camera HDR modes. In fact, the A99 has far more jpeg features like multi-frame noise reduction. I never used these features often, but I used them on occasion, and there will be moments where I miss them.
I also found the function button and menus a bit more intuitive on the A99. The D750 has a customizable "my menu" but it requires more button pushes and doesn't work as well as the function button on the A99.
Steady shot -- This is a mixed bag. I have found Nikon VR lenses to offer far more stabilization than Sony Steady Shot. But Steady Shot does have the great strength of applying to EVERY lens. This certainly allowed me to expand my glass with lenses from the 1980's and 1990's, fully secure in the knowledge they would get some stabilization.
And speaking of those vintage lenses...... One of the great things about my A99, was my Minolta 200/2.8. A lens I bought for about $650, that delivered incredible performance. I still believe the IQ was on the same level as any modern $1500+ lens. Plus of course, it was stabilized on the Sony A99 body. Considering price, quality and size, I have not seen a truly equivalent lens in the Nikon lineup.
Last but not least, in discussing the A99 strengths -- the EVF and live view. Live view is actually a bit similar on the D750, most of the same features. But of course, it works much much slower. And it lacks focus peaking, though it does have focus magnification which I always found more useful than peaking.
But the big difference is the EVF versus OVF. For 90% of uses, the EVF is preferable. The OVF suddenly looks very dark after using EVF for a couple of years. I miss judging my white balance in the EVF. And chimping shots in the EVF is much better than chimping on an LCD.
The advantages of the D750, compared to the A99:
High ISO performance --- It's not negligible. It's significant. On the A99, I had to start applying noise reduction at ISO 1600. I could get great shots at 6400, but it required very careful noise reduction. And 12800 could reach the level of barely usable, with minimally sufficient dynamic range. On the D750, I don't need any noise reduction until about ISO 6400... and 12800 still has plenty of dynamic range, and simply requires a moderate amount of noise reduction. I would say it's about a 1 1/2 stop in real world ISO performance difference.
The other biggie is Autofocus. The D750 has locked focus for me instantly in near darkness. The AF points are generously spread across the viewfinder in comparison to the A99. Focus tracking is far more consistent and reliable. That's partly the OVF as well -- making it easier to track action than the EVF.
Though in some ways I prefer the menu system on the A99, the D750 has some far superior customization. I won't touch on every menu item, but there is one big one ---- The ISO/shutter speed sensitivity. On the A99, I always shot manual. When shooting A-priority, I never liked the shutter speeds the camera chose. On the D750, I love being able to choose --- I can set a minimum shutter speed before raising ISO. I can let it shoot at 1/focal length. Or, I can let it shoot at slightly faster/slower than 1/focal length. As I have unsteady hands in general, this is the setting I like the most, just a slightly faster shutter speed. Luckily, it was pretty easy to shoot M on the A99. But I feel like I've been given a really useful Aperture priority mode, that was worthless on the A99 for me.
Pop-up flash and the Nikon flash system. For years, I heard about how bad the Sony flash system was compared to Nikon. Now I really understand it. I do use off camera flash. And the Nikon D750 makes it incredibly easy and efficient compared to Sony. I can control flash ratios simply from the body, no need to buy a F60 solely as a flash commander. And it is nice to have a pop-up flash --- to serve as a flash trigger, and also to act as a fill flash in some settings.
Finally, the lens lineup. I defended the Sony lens lineup for years, denying that it was really lacking. I was wrong. For a full frame user, the Sony lens lineup is woefully deficient compared to Nikon. First off, modern vibration reduction technology is superior to steady shot. On steadyshot, I felt I got about 2 stops of stabilization. On a VR lens, I feel I'm getting a solid 3-4 stops.
In terms of lens features... Nikon has silent wave motors on 20 lenses that are under $600. Compared to zero on Sony in that price range.
In terms of lens choices and lens IQ --- Sony has a few great lenses if you're willing to pay a premium. But for example, at a fraction of the price of the 70-200/2.8 lenses, Nikon offers a 70-200/4.. far smaller, far cheaper. But the IQ is just as good as the 70-200/2.8 lenses. Nothing similar from Sony. Sony only offers 2 zoom lenses that touch 24mm --- and they are both mega expensive Zeiss lenses. Sony only has one ultrawide -- the $2,000 Zeiss 16-35. Nikon has a 14-24 for the same price... and a 16-35/4 which is cheaper, and a very high quality 18-35 which is much cheaper. There are far more ways to assemble a high quality kit with Nikon.
My conclusion: There is no reason for A99 owners to dump their cameras in mass. The A99 remains a great camera, and continues to have many advantages over the D750.
But, considering they are somewhat similar in price, I do feel the D750 can be a better tool with higher potential. It will give the photographer a wider selection of lenses, higher potential IQ at medium and high ISOs, and a better autofocus system.
That said, the EVF system has many inherent advantages If Sony were to upgrade the A99 with many of the benefits in the D750, and spend more time growing the A-mount lens selection, it would be one killer camera and camera system.
I don't feel right posting Nikon pictures in this thread, but I'm going to link to a photograph. I just am demonstrating a few factors that I discussed in this review:
First, this shot was at ISO 11400 -- I did add noise reduction and sharpening in lightroom, but really very little. To me, this looks better than I would get with 6400 on the A99.
Secondly, this was using the newest vibration reduction technology. Shot at 200mm and 1/6th of a second. So that is a massive degree of stabilization. On the A99 with Steadyshot, it would be very very hard for me to get a sharp picture slower than about 1/50 at 200mm. But I am able to get a good number of keepers below 1/10th of a second (assuming my subject isn't moving of course).
https://flic.kr/p/p5JfrY
It's great as a hobbyist to have so many quality choices. My next camera purchase will be important for me as I still have time to switch systems before being too financially committed to Sony Emount. I could even get an adapter for the Minolta 200mm and other legacy lenses if I moved to Xmount. 
Looks like my next neck of the woods. Great photos BTW
