Pretty much what Kysafen said, but Hubert is a better comboer and faster than Asbel...but if you only attack up close you miss out on a lot of his stronger attacks and his healing. Meanwhile, Asbel is very good up close but one if his strongest (if not the strongest) attacks is long ranged (Hadou Meppuu if you are wondering what I'm talking about)
I'd say try both to see which you like more when you get the game.
Thats what I figured. I usually see people playing as Hubert rather than Asbel. I do love close combat, but when I play on Chaos, Hubert is going to be easier to play as. I'll just play the demo and see who I like better.
Evil doesn't unlock as a difficulty until you fight 200 battles on Hard or beat the game once. Chaos doesn't unlock as a difficulty until you fight 300 battles on Evil or beat the game twice. Hubert doesn't become playable until a good ways into the game.
Also chaos is kinda pushing it, but if you're really up to the challenge I suppose it's doable. I'm doing a run right now on 10x experience and 2x SP, and I have it set to Evil. It's on 2x damage but if I'm not careful my characters still get KO'd. Chaos is completely doable right now for me in that config but any mob has the potential to wipe out my party.