Well this is really using an old version of avast, in 5.0 the Behaviour Shield was very limited in its scope.
I avast 5.1 this had more rules/filters added, but the behaviour shield was in passive mode, effectively gathering data so that rules/filters can be fine tuned. This is to provide a better balance between being to strict causing false detections or too lax and not catching some unknown/zero day stuff.
Avast 6 beta has further upgraded the behaviour shield but still passive gathering data, but people are seeing it responding more. Also introduced in the Pro/AIS versions is the Process Virtualisation (Sandbox), now no matter what the risk, if you are running your browser virtualised you are at less of a risk even if something isn't stopped with the normal shields.
Interestingly the free version of avast 6 will have an Auto Sandbox feature also (not the full selective process virtualisation of the paid versions), but this based on other information, unknown, unsigned, suspicious (behaviour shield, etc.), location run from (USB, external, etc.), will trigger a dialogue box recommending you run this in the sandbox. The user has the choice to do this or select another option.
So yes I believe avast 6 will be better.