I've used both before. Free Avast v paid for nod32 though. I prefer nod32, it updates silently. So quietly I have to check manually that it has updated. In comparison you get a window popping up when Avast has updated.
The best comparison to make would be to try out out the paid for versions of Avast and nod32 but I haven't done that with Avast.
You can get a trial version of nod32
here. I think the Professional version of Avast has a better updates system, whether it is silent I don't know.
All in all I prefer nod32 over Avast for ease of use, aesthetics, effectiveness and OMG
ItUpdatesSoSilentlyness.
Your best bet would be to try the nod32 trial. nod32 has never interfered with my gaming. I don't game online and I don't know if you do but I doubt nod32 would interfere with that at all. Potentially the update window for Avast could cause a game to halt and switch to the desktop. This would be pretty infuriating.
I'll end my post by saying I heavily regret not getting a 2 or 3 year licence for nod32. It cost me £23 for 1 year, but 2 and 3 year licences cost £35 and £46 respectively (for one PC mind). So had I paid for 2 years I would have saved £11 and £23 for a 3 year licence.
Next time however.