i have a square deal, 550b, and had (and still use) a 650 that my brother currently owns. i have used all of them extensively. they are all excellent machines.
if it is going to be your first press, i suggest the 550b. you need to learn the system, and the 550 is alot of press which will do everything you could ever want it to do. if you decide later that you want to go to a 650, the resale on these are excellent.
if i had to choose one, either a s.d., 550, or 650, it would be the 550. it offers the flexiblity in use of dies, and the 550 offers the maximum amount of control out of the 3.