I have Amplitube that came with RW standard, Amplitube Live, Ampeg SVX, Guitarport, and Gearbox. As you all know by now I like hard drivin crunchy type music. I cannot achieve these sounds using any of the Amplitube stuff, at least not to my liking. Also there is so much latency when using the Amplitube stuff verses the GP and GB stuff.
Breakdown for me...
Amplitube will give you more natural way more cleaner type sound that GP and GB ever will plus sounds like a real tube amp. To me it's a more 70's type of rock sound such as Hendrix and such.
Ampeg SVX: Awesome bass sounds but with it comes the latency problem. Just fiddle with the ASIO control panel and you can get the latency to go away (this goes for both the Amplitube 2/ Live and Ampeg SVX).
GP and GB will give you more crunch than you'll ever need plus you get bass and vocal setting that are cool to have without changing over from one to the other.
Really depends on the sound your looking for whatever project your doing. If your looking for a clean tone, go Amplitube, if you want better crunch guitar go GP or GB. Again it really depends on you though you just have to open her up and try them both out.
See my post on the comparison of both Amplitube and Gearbox here>
http://www.sonomawireworks.com/riffcaster/post.php?/7115