Hey Sapient!
Avadon was my first Spiderweb game and I was enchanted by the political intrigue and charming characters. When it comes to Avernum I've only played about 3 hours of the demo, but the excellent writing is still there. However it has become clear to me that I'll be talking with my party members much less, if at all. Furthermore while the global reputation system I've read about means my decisions will affect the world, I think it will be less so than in Avadon.
However, I'm really enjoying it. The dialogue is funny, the world is huge, and it really is an interesting world to explore. I'd suggest trying out the demo like I did, then get it on Steam when you make your decision!