I don't want to die slowly, I don't want to decay; I want to be chosen, I want to be made
I'd say the first one was harder, because in KH2 you have a lot more abilities, plus the reaction commands which basically give you the chance to counter his common combos. In KH1, you can't get as many hits in as easily as you can in KH2. It's possible to beat Seph in KH2 at level 50, whereas in KH1, you have to be at least level 80 to have a chance.
I don't want to die lonely and weary of life; I will not be earth bound, I'm goin' to fly