I have both the 50 Summarit and latest 50 Summicron, and find them both really great. I have been using the Summarit more lately because it is slightly smaller and fits nicely in my smallest waist pouch using a M9. So it is a nice small stealth package, and easy to carry.
I get occasional moire on small distant buildings etc with it, which I think means it is resolving exceptional well, even at large apertures. But aside from occasional moire, I have been really happy with the sharpness overall. Also with my recent M9 calibration at Leica New Jersey, the focus is bang on. I also find the 1/2 F stop less not much of an issue indoors, unless it is really dark. You are basically talking about 1/45 vs 1/60 of a second type ratio wide open depending on avail light and ISO.
Irwin Puts has a good article on the entire summarit line on his Leica Lens web page, which indicates the Summarit MTF chart at 2.5 is in between the Summicrons 2.0 and 2.8 MTF charts. Which essentially means he feels they are the same, but without the F2.0.
Lastly, in lightroom I find all Leica M9 shots can sharpen a bit with fantastic results. Even their ASph and APO lenses.
Anyway, I am very happy with mine.
All the best in your decision.
Dave