In my opinion, I would buy the Iphone 4s, it's newer then the 4 and it has some important upgrades. Some upgrades include; better camera, faster (software in general). The 4 and 4s are almost the same, but the 4s is superior and is worth another 100 dollars. So if I were you I would buy the 4s.
*****BUT, you honestly couldn't go wrong either way, they are both great phones. As for the carrier (sprint), I don't think that it matters that much. Hope this helps