Helen, if you can find a clean 21f3.4 go for that one. The center sharpness of that version is among the best ever! When Leica made the first ones of these back in the 60's - they were sent to the microscope division for testing and came back with the note "There is something wrong with our equipment - it shows center sharpness approaching the theoretical limits" Supposedly Leica's note back was " Good, that was we were aiming for!".
The 21f4.0 is nowhere near as good. If you are looking for a screw-mount 21, you are better off with the Voigtlander 21f4.0 instead (and it also uses 39mm filters).
The 21f3.4 can be tricky initially, the edge fall off, the somewhat cramped fit of the aperture ring against the hood and the odd 48mm filter size - but once you see the negatives - all that is forgotten!
Look for one with a # above 2 500 000. Schneidar/Leica changed the coating slightly and it did improve the quality somewhat. Not a major step, but just enough that it is visible in contrast.
Of course, the true master with this lens was Jean Loup Sieff - goggle his site and drool!