http://www.pctorque.com/sager-5720-gaming-computers.php
Differences are listed there. However, it's just the way of referring to a particular build. There's no *real* difference between the 2- they're the same computer, just different specs.
If you pick the lower model and then upgrade some of the items (WSXGA to WUXGA, HD, RAM, etc.), you've all of a sudden made yourself the higher model. Nothing to worry about really.
As for which you should prefer, you should probably think about what you
need your computer to do and how much you're willing to spend towards what you
want it to do. Then read around for awhile. You'll be better off in the long run if you do some homework and don't rush into something just because it's got fancy lights and a can do the Kessel Run in less than twelve parsecs.