I'm not at the end. But right now I'm using Lightning, Fang, and Vanille. Lightning and Fang work well together. It doesn't really matter who you use the most, since everyone earns the CP, regardless of whether or not they fight.
Same, but I switch Vanille with Hope sometimes. Funny thing is, in around chapters 2-6 you always really wanted to play as Snow, since he was by far the most powerful, but now I don't even use or LVL him up.
You get access to Battle Team after having all your allies meet on the Palamecia. For Staggering, two Ravagers and one Commander is the best way to go. If all three can use Ravager, then customize a Paradigm to all three, and switch to that once the Stagger gauge is about half or so, depending on how fast you can fill it overall. It should deplete at usual speed as if you were still using a Commander. Snow is also an excellent Sentinel.
If you haven't watched it yet. http://www.escapistmagazine.com/videos/view/zero-punctuation/1569-Final-Fantasy-XIII Zero Punctuation's review of the first five hours is hilarious. Even if you like the games he hates, he still makes it funny.
I found that for their basic roles (not when they unlock every role) that Hope is primarily a Medic, and possibly unrivaled as the best healer in the game, but also works defensively as a Synergist, having access to both Protect, Shell, and Veil where Vanille works the other way using Saboteur's Slow, Poison, Curse, etc. I found that Hope has really low HP. I'm finally at Chapter 11 where it allows you to really customize and branch from the main story, and Hope usually dies in 1-2 hits even from the easiest monsters. I would suggest maxing out two Platinum Bangles or something similar to buff up his low HP. Or you can opt to use Vanille who is a little bit sturdier. I guess it's all on how you play, offensively or defensively. I do wish Vanille had access to Protect and Shell (earlier) since those tend to save me more than just using all of the debuffs on the enemy. The more defensively I play, the less stars I get, and crappier drops, but at least I survived the battle. The biggest dilemma I have is whether to use Lightning or Fang as my Commando and I've found that Fang does better, while also having access to Sentinel. I primarily use Lightning as a Ravager or a last desperation Medic. Dual Commandos are fun for maximum AoE damage with Blitz. Snow is an absolute tank. I've hardly leveled him up and he already has 2000+ HP, where the rest of the team is just getting to 1500 with Titanium Bangles. The problem I have with him, is he does hardly any damage as a Sentinel, and as a Commando he does average. The Chapter where he and Hope teamed up, the battles dragged on forever compared to Lightning and Fang. I can't wait to have my Synergists double as Saboteurs and vice versa.