If yiu're happy to spend $600 or whatever on an iPad in the first place, and you say you like it so presumably do actually use it regularly, why sell it 3-4 mons before its replacement (which may offer nothing interesting as an upgrade) just to save $50?
Keep it, enjoy it, and when it's replaced it'll maybe end up having cost you $150 in depreciation. But at least you've then had a year to enjoy it.
Incidentally, i've never noticed a sharp drop in the price of things like iPhones when they've been replaced. Sure, there's a lot more of them, but the price never seems much lower than secind hand ones have been selling for months before.
David