I have both. I bought the summicron not long after it came out with the intention of selling whichever one I liked better. I have both still, because they are different enough to make it worth having both. As the others have said, the 75/2 is very similar to the 50/1.4 ASPH -- very very sharp, and there is almost no difference in character between the different aperture settings. This is not to say it is harsh -- it has a very lovely look as well, but it is quite neutral...it does not add or subtract anything from the scene in front of you. It is smooth and sharp and shows little vignetting, color fringing or crazy bokeh. It is compact and very easy to handle.
The 75 summilux is a lens with more of a dual character -- it is softish and romantic when it is wide open, with a very classic look (but not the super softness of the oldest lenses like the summarits. It is still somewhat sharp wide open at the point of focus.), but when you stop it down to f/4, f/5.6 or f/8, it is as sharp as the modern lenses. This is nice, because it allows the lens to do double duty -- it is a painterly soft lens wide open, but a very modern super sharp lens at medium apertures. The biggest penalty for this is size and weight -- it is a rather heavy and bulky lens. Not much worse than a Noctilux or 90 summicron or 135 elmarit though. It works better with a Leica grip or built-in grip case, however. It also has some focus shift, which does not show up much on film, but is readily apparent on an M9...at least if you go looking for it.
I would not really characterize it as a great low light lens however -- sure, it can be if you get everything right, but since the DOF is so shallow at f/1.4, and it is a rather long focal length, it is harder to get sharp shots at slow speeds than it is with a 50mm f/1.4, or especially a 35mm f/1.4 (the best low light lens in my opinion). It also has some color fringing wide open (more in the bokeh than at the point of focus), as well as some blooming (but this can look lovely...the sort of glowing bokeh). It is not a technically perfect lens, but that is why it's look is so nice. Based on what you are after, I would say the Summilux is probably a better choice.
By the way, the version 1 and version 2 are identical optically, so don't base your decision on that -- just find a good copy. Additionally, I think the 75/2 is usually a better choice on digital, as it displays less focus shift and color fringing, issues which are less of a problem on film.
Some shots:
75/2:
This will demonstrate the sharpness that you get with the 75/2...this is the 100% crop of the above photo. I believe it was around f/8 to f/11:
And some 75mm summilux shots (all film)