I played AT for a few years, but by the time ATG came out had given it up and returned to TOAW, and never got into it again.
From memory, AT is much less "historically rigid" than TOAW - by which I mean things like the units are more abstract and flexible - eg you can take a unit, strip out all the tanks and make an airforce.....
TOAW locks you into the units that are there, the composition they have, the organisation they had, etc. In some cases this is a bit TOO strict (eg in a large scale game divisions should be able to be shifted between corps HQ's, corps between armies, etc...), but when you are playing a scenario at a smaller scale it is a much better simulation than AT - where you are free to move strength around as you like.
I found AT to be perhaps a step up from the simplicity of say Panzer General, but utterly lacking the depth, subtlety and immersion I get in TOAW.
I know that not everyone shares my somewhat rigid interpretation of history and demand for relatively strong limits on what players can do - and so ATG will appeal to many people precisely because of the features that I don't like - viva la difference!
