Thaumcraft 4.1 definitely. Better documentation, easier to get into, and very self-explanatory with minimum "just a thing you have to know about it." Ars Magica 2 while cool, is more about the pvp which I find less useful in the early game as an iron sword can just pretty much do anything you want in terms of pve. Also, if you get too into Ars Magica, you'll find yourself getting too much of an attribute which might bite back at you later on.