915 vs 901 ... 901 can handle to about 280-300hp/torque. 915 can handle closer to 450-480hp/torque.
901 was used on late 60s to mid 70's (74-75) 911s and all 914s. It's a 5-speed aluminum-magnesium case. 5 speed + reverse. the 914 version had an 'odd' shift pattern and the lockout on 1st/reverse gears. the 911 version had a normal pattern and a lockout on 5th & reverse.
The 915 trans was used on all 911s until, i think, the early 90's when the 993 came out (G50-50/tiptronic). all aluminum case. 5 speed + reverse. Same pattern as the 911-style 901 trans.
Now, if you want cool... do a Sport-o-matic swap
(oh yeah. I've been drinking, so the accuracy of ^ statements may not be 100% true. 50% true, even.)