Hi...
I have an M22 in my 70...the car has a 12 bolt 3.73 posi. The M21 and the M22 have the same gear ratios...however, the M22 has "straighter" cut, stronger gears, thus the whirring noise in the first 3 gears. 4th gear, however, is nice and quiet, so the whirring noise does not exist in 4th gear (direct).
I do not have any personal experience with the T10 and/or the Super T10, so I cannot comment on their durability.
Regarding your question, I guess it would depend upon the rearend that you have or plan on having. Being that I have the 3.73 ratio, the M22 (M21 would be the same) is "acceptable" for city, streetlight, stop sign driving. If I had anything (numerically lower) than a 3.73 rearend, such as a 3.42, 3.08, etc. I would go with the M20 and/or a Super T10, as the M21/M22 would not be fun to drive in stop and go traffic. I do know that the T10's had more gear ratio options, so if you have like a 2.73, 3.08, etc. rearend, you might be better off with a T10 from a gear aspect. Recently, I see that you can now get "wide" and custom ratio M21/M22's as well...so I guess it boils down to if you like to sound "cool", and have more HP, with the M22 vs. the M21.
Maybe somebody with a T10/Super T10 can chime in here and give their opinion on them.
Here is a good website for the Muncies:
Muncie
And here is some limited info on the T10's:
Super T10
I went with the M22 because I have always wanted one - I love the sound of the gears whining in the first 3 gears. I also went with the M22 because of the stronger gears. The only downside is that I am turning 3,000+ RPM above 70 MPH - but, I mostly drive the car within the city, which is extremely fun!
I hope this helps you out.