I have had the M 28mm f/2.8 in my bag since the mid eighties. It saw extensive use in my film days and I loved it. The 1.5x crop factor on my digital made for a slightly weird (to me) focal length and I didn't use it much. Until I discovered how well it works for street photography, that is. I usually mount it on a Samsung NX100, dial in focus at three meters, aperture at f/11 and shoot away. Wonderful!
I have no experience of the f/3.5, but I can't imagine that the difference is huge. You could just stop the f/2.8 down and keep the wide open option for low light. The safe option would be the f/3.5 but why not get both? Just to be sure, no gear acquisition syndrome. Then again, life is too short to have just one 28mm.