I prefer the Classic. You get better looks and a more scratch resistant exterior as well as a better battery, updated interface, etc. In your case, however, it should be noted that you will have to shell out some extra dough to get a new Classic if you're selling a 5th gen now. If the difference in cost is worth it to you for those then I say sell.
The difference you'll have to make up may make it more worth it just to keep your 5th and enjoy it. You have to first decide how much more you're willing to pay to upgrade to a Classic, which is the fundamental question. The Classic is quite nice, and has improvements over the 5th gen, but they are quite similar in most regards, so it's not really going to be a groundbreaking move for you...