I went through the same decision making process, and ended up with the 550. Basically, it comes down to the question above (how many rounds are you loading per month?) plus, how many different cartridges you want to reload. The 550 is much quicker to convert from one caliber to the other (reportedly - I've never run a 650, but everyone seems to agree on that).
Brian Enos has a good run-down on the strengths and weakneses of each here: