On my machine, the new avast! 4.7 beta is as light as AntiVir and AVG. About AntiVir, if its heuristics enabled (medium or high), I can feel some impact in real-time performance.
About overall detection rates, I know AntiVir Classic is a bit better than avast! Home but I can't really feel safe with AntiVir (or AVG) since it can be configured to checking for updates only once a day (while avast! Home can check for updates hourly) but the fact is that you can have outbreak several times a day and you don't know when, especially if using on a family computer that online almost 24 hours a day. I personally don't believe in its heuristics.
In my mind, avast! Home is far more reliable than both AntiVir Classic and AVG Free in this aspect since I know my avast! can be automatically updated as soon as it has the new VPS available.