SOLDIER wins. They have an entire Compilation to show how badass they are. SeeD doesn't.

Quote Originally Posted by Suvious View Post
Sephiroth vs Seifer - Ex-Soldier bad guy vs Ex-Seed bad guy = Soldier wins!
I don't know... Seifer don't have many feats story-wise, but if we only compare what each one of them did to a Summon, then we can say that Seifer is a little stronger than Seph.
Sephiroth killed Ifrit in two strikes(the first strike was the one that stoped Ifrit from attacking Zack again, and the second strike was the one that killed Ifrit), but Seifer killed Odin in one strike.

If we assume that FFVIII Odin and FFVII Ifrit are on the same level, and if we only judge Seifer and Sephiroth's powers from what they did against these two Summons, then Seifer is stronger than SOLDIER Sephiroth.