If you think you'll be riding two up with frequency, go for the 750.
If you ride mostly solo, the 550 will be more than adequate and more nimble.
I will warn that some 550s prior to mid year 76, can have the dreaded rocker cover wear issue.
Look for F model engine serial numbers #1133172 and subsequent for engines with the factory improved covers. (S/B 500/550 #8)
Cheers,