Basically, the amp and FX models on the HD models are significantly better than the ones that have come before.   It is an issue of quality over quantity.  Twice as good? Dunno, depends on what your ears tell you.  Personally, I did not rate Pod modelling too highly before the HD but the models on this one respond much closer to real amps and that is important to me.

For plugins ... well actually the POD HD's do not do this.  They do not behave like a VST so you cannot insert them somewhere within a workflow.  A Pod HD, when connected with a USB cable, will become your PC's Audio Interface.  That is the whole extent of the routing options.  You cannot use the HD flexibly within a mixing scenario other than with an external mixing desk etc.

It is fair to say that there is some functionality on the POD HD's predecessors that is desirable but did not make it onto the HD series.  With that said, for live performance, the POD HD is more convincing.  You takes your choices.