Lots of experience with Guitar Rig, and as with any Native Instruments product it is simply high quality. I recently upgraded to GR5 and what I love most about it is that I expand its use to vocals, bass, and even drums for some odd effects.
Amplitube is still decent, although I find the general sound of anything on it to be very lacking in power, even when mixed properly with external effects such as compressor, EQ, and such.
Another one to look into is Overloud TH2. I like this one better than amplitube but less than GR, although a lot of people I know are switching from GR to Overloud, depends really on the music you intend to make.