Personality-wise I'd go for Tidus. Many people complain he's a wimp when he's actually mature and sacrificial in the end. Zell was also quite immature plus his aggressiveness and his hot temper.

Nweis granted that Tidus has maxed out the Sphere Grid, is equipped wit Caladbolg, has all Overdrive modes (and all other power-ups) while Zell has all Limit Breaks (and other power-up blah-blahs since I forgot almost all FFVIII details), I personally think Tidus would win. Just my two gil.

Lastly I'm surprised that some people still can't get over the fact that not all FF leads should have the "hardass" image.