If you are using him most of the time, consider grabbing Wakka's Triple Foul ability for Auron. It has its uses--in particular, silencing Demonoliths to prevent Pharoah's Curse or whatever that nasty counterattack is, is handy, or putting Tonberry's to sleep. I actually worked Auron backward through Wakka's grid for awhile after finishing him up on the second half of Tidus's--a decent case can be made, I suppose, for running through an entire grid (especially if you are looking to pick up certain abilities), but typically the good +4 stat boosts are in the latter half of each of the individual portions.

As for favorite weapon, I have to admit I rather liked the damage Caladbolg was doing in Tidus's hands, but since all have Break Damage Barrier, '1MP Cost' > anything the other weapons can offer, so I'll go with Nirvana. Plus Yuna > the sum of all other FFX characters and dolls for a weapon is just totally stupid.