The way I see it is this, we dont know how well seifer is equipped via his GFs. From a gameplay perspective, we dont know what hes junctioned with, or GF abilities equipped. Theres 2 ways to see the confrontations with Seifer:

Seifer is equally geared as Squall in terms of magic and GFs. This is how you actually see it in the game (or at least, I guess supposed to see it), and in this way Squall typically whoops Seifer with no trouble. Like someone said, even in the 3v1 fights, if you have spent time gearing up with Squall, Seifer 1v1 still wouldnt beat Squall. In this situation, you could assume that once Squall becomes a SeeD, his power grows faster than Seifers.

However, if you look at it from a non-gameplay perspective, Seifer is probably stronger in terms of 1v1 fighting. Squall beats Seifer in Delving, yep, but I personally think that after that point, Seifer becomes stronger than Squall. It takes a group of 3 to defeat Odin, and Seifer does it in 1 blow. If thats not enough, every time after Delving you encounter Seifer, you fight him as a full group. GFs and Junctions aside, if Squall was stronger than Seifer he would probably opt to whoop his butt personally, without dragging the others into it. By the time disk 3 rolls around, Squall would no longer be able to stand up to Seifer 1 on 1, and he must rely on his friends. Which is actually the way the story is supposed to work. The whole strength in numbers routine. Individually, Squall might be weaker than Seifer, but by Disk 3 he trusts his allies to watch his back, and you could argue that because of his friends, Squall is a stronger person

Anyways, just a rant. In terms of personal power, Seifer wins in the end imo. But storyline wise, most would prefer being surrounded by trustworthy allies who fight for you no matter what, which is where Squalls strength lies