I think the difficulty between the two series is pretty much equal. However, I found that some encounters were harder in the Exile series than in Avernum. For example, I spent several hours getting to the slith king in Exile while in Avernum I probably spent about twenty minutes. Exile 3 in particular is quite different in my opinion. I thought the rakshasha battle in Execa, Black Halberd dungeon, the undead infested inn in the Midori province, and the final battle with Rentar-Ihrno were all harder in Exile III than in Avernum 3.
As for general combat, the lack of spells in Avernum makes combat in a bit harder. In Exile you could simply bombard the enemy with divine thud, firestorm, wall of blades, and summmoned basilisks.