Either is a good choice--just make sure you get the Classic Turret with auto-indexing. Look for the indexing rod running from the turret down to the ram.
A lot of guys seem happy with their 550s, but I would maybe give a thought to the 650--literally just to get the auto-indexing. I can't remember which of the Dillon presses is upgradeable to which. I'm sure one of the Boys in Blue

D) will be along to tell you.
Which to get depends on how much you want to shoot, and how much time you want to spend reloading. Fred will be along to tell me I am insane (again,

). I concur that either the 550 or the 650 is money well spent, I'm just too damn cheap to spend the money!