I feel that Demise was just thrown in at the end. Ghirahim actually followed you throughout the game, and he was more of a villain to me than Demise was. Demise wasn't in the game for long, and didn't have any development. Not to mention, the fight with Demise was pretty underwhelming compared to the battle you just had with Ghirahim, in fact all three duels with him are more fun and challenging than Demise's. Fighting Demise was like fighting a stranger, while fighting Ghirahim felt like I was finally putting an end to a maniac's plot for world domination. I love this game for letting me refight bosses at Lanayru Gorge because I would be lost without redoing the Ghirahim duels.
Ghirahim > Demise