You can always upgrade to OA Premium, continue as you are with OA Free/Avast Free/on-demand scanners, or layer your lines of defense for better protection.
PrevX does offer a free version with a paid SO, so you can look into that if you want. Ultimately it is your choice. As for FW, OA is very strong; here is the comparison chart for the difference between OA Free and Premium: http://rus.tallemu.com/free-firewall-protection-software.html - they do specials so their prices will drop at those times. I prefer PrevX w/SO over OA’s Banking Mode for my needs, and as I said I prefer to layer my defense for maximum protection, and the software does not conflict. Only you can make the decision as to what is right for you.