There are free versions of AmpliTube, Guitar Rig, and Line 6 POD Farm. They're quite limited, but they'll give you a feel for the programs.
To me amp sims are like guitars, they're all different. I have pretty much all of them. I use AmpliTube more than any of the others, but all of them make unique sounds the other ones can't make. I do like the Scuffham, BTW, good stuff. The guy who coded it did the code for the Marshall JMP-1 IIRC.
I'm just starting to dig into TH2 and want to check out the full version at some point.