I wouldn't upgrade from the 508 unless there are specific features (like pre-amp outputs) that you think you might need in the future. I don't think going from the 508 to the 608 is meaningful, but I haven't studied the features closely.
Accessories4less has it for $209.99 plus shipping, which seems like a pretty good deal.
If I was still a college student, that's the one I'd get. It's hard to find any decent used gear that cheap, and this one has all the codecs and switching you'll probably need for quite a while.
