You'll want to do some homework on costs of conversion changes, primer swaps, etc. The Dillon 650, in my opinion, is only worth if IF you're going to get the optional case feeder! Otherwise, you still have to put in a new piece of brass, and then you may as well have the 550 and the reduced costs & time of caliber conversions.
What's your desired round count? How many different calibers do you want to load? How many places for dies do you want/need? How much room do you have (you'll need some height if you want a case-feeder attached)? Etc. etc.
I'm not dissing anyone, but a lot of people say they 'want' or 'need' the auto index, I thought so too, but after loading a while on my Dillon 550, it doesn't matter to me, the speed's not any different, especially if you don't have a casefeeder for the 650...
Also, there are quite a few issues documented with Hornady's priming system. Dillon's not perfect, it likes to be clean, but doing a bit of homework may save some heartache down the road...