Depends a bit on how and what you're shooting.
I've used the Rokkor 40/2.0 on an M3 some years back (had a Minolta CLE and lenses and then bought an M3 without a lens).
When composing with the 50mm framelines you simply get some extra image when shooting, which you can always crop off if necessary. Real nice when shooting 'street' (or reportage as it's called nowadays) since sometimes you find something extra along the edges of your frame that adds to the shot.
Now, when you want to show your shots are uncropped (it's an issue with some photographers to not crop ever) and you want precise composing, you would need a finder on top and that would slow you down, at least a bit.
The Rokkor 40/2.0 makes the camera pocketable (large pockets though) and takes easily found 40.5mm filters. The Summicron-C is just as big but takes Series VI filters, much harder to find. The CV 40/1.4 isn't that pocketable at all.
Hope this helps.