Hardware packaged, software driven amp sims are always going to be at a slight upgrade disadvantage. They do thave the advantage that the initial price can lead to better research, but thats often pushed out for the hardware placebo effect.
Have a look at the Studiodevil. The free version is great, might be WAY too trebly for some people, but it is authentic
http://www.studiodevil.com/home/
The neat thing about amp sims is that, because of the educational research nature of the modelling side, the freeware is every bit as good or better than the payware in terms of the simulation of behaviour
One BIG spot people are getting wrong on the plugin side is not realizing they need to get in with a direct box. Plugging a passive guitar pickup into a line input isnt going to do much
Another one is blanket statements.
Man of the amps out there are just cheap crap. To think you can beat that with a sim is ridiculous.
Another is application. Are you just trying to make the guitar louder? then you dont need a sim or an amp.
Trying to get the sound of an amplifier working in its linear range without the coloration or phase issues of a microphone? This is no problem for an amp sim.
Modelling the behaviour of a tube stage being driven into its linear range? THIS is the holy grail. Where it all gets tricky. Many actual, high dollar tube amps fail this as badly as most amp sims