This is what I was originally thinking, until I started reading reviews that said there was considerable IQ difference between the VC and non VC models.
I've read enough reviews to think there is a difference, but not having used the non-VC version, I can't say whether it is "considerable" or not. I do know that proper shooting technique makes a big difference in image quality, and a lot of people don't do all they can to maximize IQ at time of capture. If I want the sharpest image I can get, it means using a stable tripod, preferably without using an extended center post (although my height vs. my tripod means I usually do wind up extending it anyway), turning the VC feature off (while using the camera on a tripod), using Exposure Delay, and using a remote release. For some people, it also means turning on Live View, using the magnified view, and fine-tuning the focus; these last steps are not something I do in most cases; I find the hassle outweighs the potential benefits. YMMV.
So in you experience the VC models IQ would be better than my kit lens?
In truth, I can't say. I've never used either of the kit lenses you have -- the only kit lenses I have are the 18-135 non-VR and the 18-55 non-VR, and I've hardly
ever used the latter. According to what I read, the largest difference between the VC and non-VC versions of the Tamron are that the corners are softer, especially wide open, on the VC lens. I can attest that they are, indeed, rather soft, but so far that hasn't bothered me. I don't generally put subjects of critical interest right at the edge of the frame, except by accident. I also don't shoot test charts or pay much attention to the numbers others get as a result of doing so. I don't photograph test charts; I'm more interested in how well a lens performs in the kind of shooting I actually do on a regular basis.
There have been some reports that newer copies of the VC lens are better than earlier ones with regard to the IQ issues some have reported. I don't know whether that's true or not. Lots of people talk without really knowing what they're talking about. Others talk and
do know their stuff, but if you're just Googling reviews and skimming the results here and there, it's hard to tell who is knowledgeable and who is not. There are a lot of parrots out there, and they don't all stay in the Tiki Room!
I want both, but I really don't want to spend the $$ to rent both, when that money could have gone towards the purchase of one or the other.
That's always the way it goes, isn't it? Some advocate buying lenses rather than renting. If you take care of them and then decide you'd rather have something different, you can sell the lens and get almost as much as you paid for it. I have no doubt that would work.* If you have the cash on hand, that could be a good way to go. I don't have a credit card and wouldn't buy anything on credit except a house, so I wouldn't do that. I've also never rented a lens, though I have been tempted. If you
can do that, you might wind up paying less in the long run than the cost of the rentals.
If you're in a position where you can only rent one of them ... well, I don't know. 17mm is decently wide, but it certainly isn't 8mm or even 10mm. If you
can rent both, I'd still say get both. If you
must limit yourself to one, you'll just have to decide what's most important for you to shoot.
Just for reference, here are some of the cave photos I mentioned earlier ... the two boys you see closest to the camera in a few shots are mine. I like those images because they give a good approximation of how dark it was in there to the eye. All of them were shot at f/2.8. More than half are at 17mm. Shutter speeds ranged from a high of 1/60th (on the darkest shot of my little boys) to a low of 1/4 of a second (on the one with the blue-lighted formations in the foreground).
*From eBay, I once bought a kit that included a Nikon N2020 film SLR, an old 28-300 push-pull zoom, A Nikon 50mm f/1.8 AF-D lens, and a camera bag. I wanted the 50mm lens. I paid just about the going rate for the 50 for the whole kit. I got it, kept the 50 for myself, cleaned everything else and made really good photos of it all, then sold it for
more than what I'd paid for it in the first place. I love it when that happens!
Here are the photos I mentioned:
Caverns 4323 by
Scott S. Baxter, on Flickr
Caverns Stitched Panorama 2 by
Scott S. Baxter, on Flickr
_DSC4346 by
Scott S. Baxter, on Flickr
_DSC4341 by
Scott S. Baxter, on Flickr
_DSC4335 by
Scott S. Baxter, on Flickr
_DSC4220 by
Scott S. Baxter, on Flickr
_DSC4219 by
Scott S. Baxter, on Flickr
_DSC4216 by
Scott S. Baxter, on Flickr
_DSC4144 by
Scott S. Baxter, on Flickr
Caverns 4138 by
Scott S. Baxter, on Flickr
SSB