Richard is correct in the 41x0 and up. Closer evaluation as the information is coming out about the X4xx and the x5xx series is that they appear to be enhanced (4 wheel drive , a little more weight, a few other enhancements and more price), 400 series garden tractors.
The 41x0 and up series are getting improvements that coul make them very desireable. However, I also would wait for the first or second year bugs to be worked out. As they are adding electronically-augmented HSTs on some of these and that potentially could have owners experiencing a few problems initially.
But then again, i bought the 1996 Plymouth Voyager and it did pretty good, they replaced 3 gas tanks due to a problem in 4 months, and had to replace the filler necks and some electrical harness due to a discoverd potential fire hazard, over the next two months, but I did run that vehicl for 4 years.
Nuru