I try to keep all my characters SP levels to be the same or only one or so apart that is the best way to make sure all your characters are capable in battle and that if you bring in a reserve member they can hold their own and do a decent job that you have designed them for e.g Lulu Black Magic Yuna White Magic, Aeons, Auron Powerful physical/debilitating attacks. However when all the characters have maxed the sphere grid I personally like to use Tidus, Auron and Wakka they are the three coolest charcters in the game!!!