This is topic which usually brings about debate. For me it's easily Gundam Wing. I prefer all the characters in Wing because they don't whine as much as they did in SEED. For me SEED was too immature in a manner of speaking. The war they fought wasn't very realistic at all, with crying teenagers fighting in wars they'd rather not. Wing was about teenagers who wanted to bring about pacifism but realized that in order to do so they would have to fight for it.
I guess I just preferred the more hardcore characters in Wing than the softer characters in SEED.