If the Nikkor is clean, keep it. You really aren't missing out on anything. I used to have a Summicron with my M3 but I moved on and I don't miss it. Frankly between the two i would rather have the Nikkor. I have several 50's now but I use two old Contax Sonnars, one coated and one uncoated, with an adapter. Basically the same as the Nikkor. The Summitar that Thomas mentioned is a nice lens too.

Remember that the limitation lies within not without.