Kingdom Hearts I, CoM and II are a joint project between Disney and Square-Enix. Square-Enix, previously known as Squaresoft, created the FF series.
Various FF characters are in KH, to name most:
Cloud(FFVII)
Leon(FFVIII)
Aeris(FFVII)
Yuffie(FFVII)
Tifa(FFVII)
Cid(FFVII)
Selphie(FFVIII)
Seifer(FFVIII)
Fuujin(FFVIII)
Raijin(FFVIII)
Vivi(FFIX)
Setzer(FFVI)
Tidus(FFX)
Wakka(FFX)
Yuna(FFX-2)
Rikku(FFX-2)
Paine(FFX-2)
Sephiroth(FFVII)
Moogles(Most FFs)
Almost all the not KH only enemies are from Disney. In fact, the only FF villain is Sephiroth, and he's an optional boss. An incredibly hard one too, but, well if he wasn't I'd complain.
Oh, my votes for FF. I dunno, guess Bahamut tipped the vote for me. It was an INCREDIBLY hard choice. Mickey Mouse almost tipped it to KH, but, well I just prefer FFs a BIT more. Just a wee bit.