I have the DS version and I'm definitely keeping it. If you're thinking about trading it into Gamestop, I say keep it since you'll probably get nothing for it, maybe like $5-7 at the most, which is not worth it.
I'm going to pick up this PSP version, but probably later on. I initially thought it was $19.99 but the price was changed on Gamestop's website to the current price.
I know Square-Enix usually charges more for their games, but seeing how Legend of Heroes Trails in the Sky, 3rd Birthday, and Dissidia 2 is $29.99, FFIV is kind of a hard sell for me because I already played the game so many times and it's just an enhanced port of a port!
$19.99 would've been a great price. I wish SE didn't change it.