Ginge, cant comment on the M3 vs RA but can about the g/box as i've not long sold my 1996 WRX RA.

Only the early RA's (pre 97 - < check that though) came with the short gearing. Go for a later one...imho the gearing was a pain in the ass, 120mph on the limiter and the acceleration didnt feel alot different, was worse on fuel etc. Only bonus was being able to stick into 5th doing 30mph lol!

If you're going for the impreza, deffo agree with getting one with DCCD so you can have fun in it. Non DCCD one's, compared to a rwd saf, are a bit boring to drive if you like to mess around a bit (only my opinion of course).