OK just my opinion, I have a lot of 1st hand experience with C4's and none with Powerglides. That said:
Pros for the C4:
3 speeds
2.42 first (better with your 3.55's) vs. 1.76 in PG
cheap, plentiful performance parts, easy to rebuild
Lighter
Made by Ford
The guys I know running powerglides whether in GM or Ford vehicles are making really big power (so are you if you have 700hp) and run that trans in racecars where they know EXACTLY what RPM they want when they hit the traps. All things being equal, 1 shift takes less time than 2 shifts, too.
I think generally it makes more sense in a performance manually shifted auto trans streetcar to have a 3 speed.