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For me personally 2;
Short answer - The ability to shot 35mm and the viewfinder magnification.
Long answer - First the magnification on an M3 0.95 + glasses which I wear was not the best, I couldn't see the whole 50mm frame lines and I don't usually shot too much 50mm, this bring me to point 2, which the M2 has frame lines for 35/50/90 and the M3 not. So you need (goggles) to shot anything wider than 50mm on an M3. The M2 also has the standard 0.72 magnification so even thought I can't see the full frame lines on 35mm it's much easier to my eyes and glasses. And in comparison the M2 feels lighter which is a plus as Leicas are tanks.
YeahNerdz-
I found the best compromise for glasses was a stupidly expensive diopter with the appropriate magnification for your required correction. Then remove your glasses while looking through the finder. It helps if you go to a shop that has a box of them and try them out until you find the right one.
Of course that wouldn't change the magnification issue with the M3 vs M2 but it does help with focussing.
Airhead
Out of interest why trade M3 for M2? What are the benefits?