I used to have Avast and it somehow tend to let malwares get in my system. That happened to me several times and took me long time to fix my system. Never using avast again! Ive had AVG and Malwarebytes for quite long time now and no issues at all. All up to you but with the experience I had I will not recommend using Avast.
I use to use AVG-and upon having a severe speed issue, came here (a while ago) and was told to use Microsoft Security Essentials. I have not had Speed Issues, Malware nor Viruses since.