I'd say, based on reading all of this thread so far, that the best advice is to slow down and read up. You don't sound as if you quite know all of the market, what's available, what suits your needs, etc. It will be worth it for you to do a little more research as to what the various DSLR, SLT, and mirrorless options are to make sure you get one that works best for you. And be very very careful when browsing camera forums, especially brand-specific ones, because you are going to get a lot of contradictory comments that can be confusing - lots of 'this brand stinks' stuff countered by tons of 'this brand is the bomb and YOU stink' stuff. Try to steer clear of any threads that try too hard to say why one brand is better or worse than all others, because it simply isn't true.
First, there's not going to be any significant differences between the DSLR/SLT products by any of the big brand names in the market: Canon, Nikon, Sony, and Pentax. Each of them may have entry level offerings, medium level offerings, and higher end or pro offerings, and pretty much each of them will be comparable in any given category. There may be specific features or advantages with each brand - larger lens selections, better backward compatibility, specialized features like stabilization in body or video specs...but for the most part, they all tend to balance out between their advantages and disadvantages. And feel is important - see which ones fit you best, because they're not all the same.
Second, the mirrorless cameras are indeed perfectly capable of matching the IQ of DSLRs - in fact, several of the mirrorless models use the exact same APS-C sensor, so there's nothing IQ-wise that makes them any different. The sensor you'll find in many Nikon, Pentax and Sony DSLR/SLT models is the very same one in several Sony's NEX models, for example. As with anything, the mirrorless cameras are a package of advantages and disadvantages that have to be weighed against your needs and likes. They are indeed much more compact, lighter, and very capable, while they also inevitably suffer from smaller grips and ergonomic differences, smaller buffers, and less battery life, and use different focus systems.
As for the low light needs, again - the brand won't be a factor here - the same lovely low light photos can be taken with any brand of DSLR, SLT, or mirrorless model. What will make the better results are the lens used, the photographer who takes the shot, and the processing technique or software they use. Brand name is just about the LEAST important factor in how good the IQ of any of these cameras is!
Take the time to research the sensors - look at the mirrorless models in both M4:3 sensor format (Olympus and Panasonic) and APS-C format (Sony, Samsung, Canon), and see the relative differences or similarities in performance compared to DSLRs and SLT models. Try out some of the Sony SLT models in person to see if you like electronic viewfinders or not - again, they have some big advantages and some disadvantages compared to shooting with an optical viewfinder, and you may love it, not like it, or be completely adaptable to either. Once you start honing in on systems you like, make sure they have the lenses you'll want to use...most of the DSLR/SLT systems have 100+ lenses available to them, between the manufacturer lineup and third party lens makers, and even the smallest of them will stretch to over 300 lenses available if you include older autofocus lenses that can be bought used and still work fine. The mirrorless systems on the other hand all have much smaller lens selection that will autofocus - still might be plenty to meet your needs, but the largest of them (M4:3) only have 30 or so lenses available and some are so new they only have 2 or 3 (Canon) - Sony and Samsung both have 10-20 lenses for their mounts that will autofocus. But a side perk of these mirrorless systems is that if you're willing to forego autofocus, they can actually mount tens of thousands of lenses from nearly any mount ever made in history.
Good luck, and hope any of that helps!
