I like AVG because it's very light on real-time performance (but not as fast at on-demand scanning, avast! is faster) but I don't really know why, I feel that the avast! engine is so stronger and ahead the AVG engine. For example, avast! has no so-called heuristics in its on-demand scanner but avast! does a better proactive detection than AVG that claim it has heuristics, avast! also has a better update mechaism and other robust features (e.g. HTTP scanner, boot-time scanner, etc.) than AVG. For overall solutions (not detection rates), avast! is unique and better than AVG (IMHO).