Dungeon Siege is basically one long road that you run along killing stronger and stronger monsters while upgrading your equipment (a review I read called it a 'rail shooter'). Groups of monsters are too tough to take on with a small party, so at least in the demo I was required to inch up to draw off a smaller number of monsters, then back away, fight, repeat ad nauseam. Apparently the multi player portion is better, but I've only seen a couple comments about it.
Divinity is single player, but has much more open game play, and many more quests / quest variety.
Both games have demos. The beginning (~10%) of Divinity (what the demo includes) and the very end are a little more hack and slash than the majority of the game, with fewer quests. It should give you an idea of the game, however.
I got Dungeon Siege for Christmas. I suppose I should open the box and install it sometime.....
