Alex is spot-on... Exactly. As far as sound, the "classic tube amp" sims are very useful and the ability to mix & match cabs, mics, placements, etc are also very useful.
I also agree with the comment about "heavy distortion" - it CAN be bright and harsh. You just can not replicate the amount of air that moves when laying down trax with a JCM 800 and a 4x12 cab (or like rig).
FYI: I have found that the tweed and "classic" amps sound fat and heavy once you "replicate" the real world recording process: Double track them, switch gtrs and pickup combos and play with the cabs, mics, room, etc. Pan 'em hard left and right, and it's close... All in all, Gtr Rig is the best I've heard in a sim...
I never use Amplitube anymore. Dated for me.
HOWEVER, here's a great "analog" alternative - and it works very well: Axetrak -
http://www.jlhproducts.com/axetrak/
I own one and I have used it on the road, hotels and other "spaces' to track ideas. In fact, I had a great riff going and since the laptop was up, I doubled the track with Gtr Rig for more "color" and it was way fat (for what it is). Of course, nothing beats a great tube amp pushed to break up with the right mics and pres.
BUT, back to the AxeTrak...
It has a proprietary build with an altered 57 mic ( mounted slightly off axis & fixed inside - it never moves) and I believe it has an 8" speaker... The specs are on the website. Oh, yeah; XLR out for DI, fuse protected, pro built unit.
Even with the amp cranked, I could play my guitar with tons of crunch and have a normal conversation with someone in the room right near the AxeTrak. Fully cranked, the box is no louder than the volume of a TV set - at the volume your parents would listen to it. Really.
What's also cool is there's a small cover that can be removed to push some air and even then, it's only about 25% louder.
If you're in an apartment or the like, this thing is a great "Analog" alternative.
Check the AxeTrak out. You may be surprised. I was.
Good luck!