I actually battled Sephiroth in 1.5 recently and surprisingly he wasn't that bad. Took me a few tries, but I beat him on proud mode with kingdom key. In comparison though, Sephiroth in KH2 was much easier.
If you're at the right level, then yeah, KH2 Sephiroth is a lot easier. I do think that KH1 Sephi is beyond more difficult than KH2 sephi. It made me so angry when I couldn't even damage Kh1 Sephiroth one bit. I ragequitted so many times when I tried that. >_> xD