You will almost certainly end up with more than one press in the long run. I have a 550B, 650, and recently got a 1050. The 550 is definitely the easiest to run small batches, the easiest to switch calibers and primers, and the most forgiving of issues while in use (because there will be). I won't get rid of the 550, even with the other two (and a C0-ax).
If you want to produce a LOT of one or two calibers, especially if they both use large or both use small primers, and you know what you are doing, then a 750 (or 1050 in very large quantities) is a great choice. If you want to load up to 500 or so at a time of your most used caliber, and also want to occasionally load other calibers, the 550 is going to be better. I would also say the 550 is better for less experienced reloaders. Certainly it is possible to start on a 750 and do fine, but... it is harder to do small batches for load development, and auto index makes it MUCH harder to fix when things aren't going right, and setup is a little more finicky for the same reason.