I've used both, and I think Avast has a better detection rate, And much more configurable.But the biggest plus are these forums, The people here are VERY knowledgeable and will help you with more than just Avast problems, unlike the AVG forums where the people who are SUPPOSE to help you are very rude and condescending and not much help at all. (this is just my opinion)
BTW, Welcome to the forums