Well detection is similar since they use same av signature. As for protection avast 8 is the winner. So if I was you I'll remain with avast 8, really just for the UI to give up on a stronger version isn't the best choice. You need to decide what you want a interface that you like or a better av(av isn't a browser or an OS that you interact daily, you only open when you scan or when you want to modify some settings/see security detail, but really you don't do this every day). As for the UI, avast 9 will bring some changes in this area.
Regards, RealNature