One is made for the iOS, one is a port. What is more important to you? Frankly I love both so you are going to have to bite the decision bullet on this one yourself I think.
This is a no-brainer: BOTH. But if I HAD to choose just one, Secret of Mana. I played it on SNES, Wii, and now iPod. It's just simply the best nostalgic RPG ever.
if you never played it, id say get secret of mana. if played in past then Chaos rings, or if ya got the loots, both
Tough call. Both fantastic games, though I have yet to play SoM on iOS, but I played just about every port out there including the SNES original, so yeah. But Choas Rings is excellent. Just good, very challenging.
Also depends on how much you are a fan of Final Fantasy series. I guess the poster is not such. A game reviewer friend says the CR is more or less a puzzle game and I don't know, I pick SoM on the day it is out.