They are both good IMHO. Just that Divine Divinity is more familiar, and will teach you the fundamentals of the game. Beyond Divinity, again IMHO, is a more difficult game with many new innovations. I think you could say playing Beyond Divinity first would be like playing the Hordes of the Underdark without ever playing the NeverWinter Nights core game.
Probably what sets them apart is that Divine Divinity is immediately addictive, from the very start. Beyond Divinity is a slower addiction, and you have to go through a steeper learning curve.
Kind of a lousy explanation, maybe someone can do better.
