It is all about what suits you best as they are all in their own way different, offering different forms of protection so you have to look at what functions are in the free version and if your needs are met. However, one thing most often forgotten is support not so much a specification/function, but no less important.
avast hasn't taken the approach of many free products, crippling/limiting the protection (exception script blocker) and the free version is virtually the same as the pro version, the lack of automated functions, interactive input requirements and the Simple User Interface rather than the Enhanced User Interface.