Avast is certainly much lighter (with respect to resource usage) than Symantec. The forum threads you're probably refering to, while valid, cannot really be considered as representative samples of user experience with the product. Remember that most people come here only when they have a problem!
We have quite a big number of server installs (even big server installs) that are running flawlessly. I believe that most of the lockup issues in those threads are in fact related to some sort of collision with other software installed on those machines. And they need to be solved one by one...
What I'd like to say is that both products (well, I'm not sure about Symantec - but avast certainly) provide a trial version - so the best advise is to go ahead and install it, test it, and see if it works well for you!
Detection wise, they should both work very well - but it's the details (and the price) that makes the difference.
Avast has a 60-day trial period which is IMHO long enough to get acquinted with the product thoroughly and see if it meets your needs...
Thanks
Vlk