Depends on what you do. I have 2, 2000 Polaris Indy 500 sleds. Mine are great, I am not out for doing 110 on a trail. If you are looking just for fun, get the 600, they are still plenty fast enough. I know I have had mine up to 65 or so, never cared to try beyond that.
My brother in law has a MX 750 or something, and that thing is pretty fast too. Polaris makes the best sleds imho. I will never use another brand. We just got about 8" of snow this week, so this evening the wife and I are throwing on our Choko riding gear and heading out on the trails (the state trail is 300 feet from my house at the end of my driveway
) I can jump on it and ride all over Michigan even to the U.P.