I think Guitar Rig is the better choice to use with synths. I've also got Amplitube and all I can think of it adding is more hum and distortion. However, it's up to the artist to think outside the box and use these things in ways not intended, look at the tb303.

Amplitube does not do anything that could not be done to a VST in Guitar Rig.

I am for a meritocratic process of applying "artist" to those demonstrating a level of finesse and expertise. Everyone is not an artist simply because they dabble in music or with art supplies yet the word is used so loosely by pop musicians that kids think it is another word for musician.

I wasn't necessarily considering the OP when I used the word 'artist'..but that discussion is for another thread.