In the 500-650 range you have a lot to pick from. I was in the spot and in the same price range! I picked up a combat ego10 with full barrel kit for $550 and im VERY happy with it! Egos hands down are one of those speedball markers that you can beat to hell weekend after weekend and it will still work!
Now with that being said you also have choices like a used Bob Long G6R ( I have a friend who is selling one with SLP tank and a Z2 prophecy want more info? PM ME
) again another favorite of mine! also have the Geo2, or Geo 2.1, BL Vice, BL Victory, Dye 10, and 11, the Empire Axe! (again I have a BRAND NEW one for sale! PM me for info)
All in all it is what makes you the user happy! the feel of the shot, how it feels in your hand all matters.