The way I look at it is how many rounds will you load in a session, and how often will you change calibers?
The advantage of the 550 is lower initial cost, less expensive cartridge conversions and less time to convert between calibers, the downside is it's slower because you have to feed cases and bullets manually.
The advantage of the 750 is you can load faster, but the downside is higher initial cost and cost of cartridge conversions.
I'm happy with the 550 because I load from 100-500 rounds in a session and have it set up for 1/2 dozen calibers (and growing) If I was loading 1000 rounds or more in a session I'd seriously consider a 750.
