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.
One of the things I really like about my 550 is that I can pull a pin and take the case with the powder charge out from time to time and check the charge.
Some of the other Dillon Presses you can not do that very easily from what I have been told.
I started reloading on a single stage RCBS Rock Chuker and really liked it but then I got into shooting a lot more than that single stage would keep up with
I now load 15 different cartridges and changing between them is just a few minutes.
