Each to their own it is a matter of choice, but the cost is small and a one off cost, and the resident anti-malware protection will I believe improve overall system protection and it shouldn't conflict with avast.
I haven't used it as a resident anti-malware scanner (already having SAS resident, you don't want two resident anti-spy/malware programs), so I have no practical experience of the resident function. However, the on-demand function works well and is relatively fast and low on resources, so I would hope the resident function would be at least as good.