I like both depending on how it's done. I actually feel it's a bit more realistic and immersive when they split up because, let's face it, RPG heroes get in to some heavy stuff. People are going to leave, split up, etc. Get other things done, have important things come up, yada yada. I really like how IX did it and I felt it really added to the story telling for me personally. Like WK said, it also gives me an excuse to use and get to know certain characters I otherwise wouldn't pay much attention to. It can also help larger casts develop if they have their own focus in a smaller group.

I can also enjoy when they give you everyone and you keep them because you get to stick with them and watch them develop that way. You get to see them beginning to end and you never have to worry about your favourites being taken away. Like Formy though, I hate it when they give you late arrival characters and you don't get to see them grow as much or have as much time with them. So really it depends on how it's done.