They're two different genres. One's a fighter, the other's a RPG.
If you want a good RPG (probably one the few on the Cube) with a nice, although clichéd storyline, get ToS. The battle system is entertaining and it'll certainly take a while to beat. Maybe around 20-30 hours. It has multiplayer, but it's not really worth mentioning (couldn't even get it to work). Keep in mind that the game doesn't have much replay vallue, after you beat the game once you'll probably not finish it a second time.
Soul Calibur 2 is a great fighter (heck, it even has Link in it

), but has very little storyline. It's a great multiplayer game, though. I've had a lot of fun with it playing against my friends.
Honestly though if you have the money, get both. They're both excellent games.