Wow...well first, you can find 101 different threads comparing GGWBtS, AGEODS ACW and Forge of Freedom. In the end, one is not clearly better than the other so dont think I am telling you that. Now, having said that, i am an AGEOD fanboi for sure...I have 'em all. I will tell you that I find GGWBtS to have the better AI. AGEOD has more involvment and detail regarding organizing your forces...much more micro. More sub types of units and such. I think this makes it harder for the AI to properly organize forces. Also, although AGEOD has more territories, I think this can create a problem for the AI as well. In the end, I think AGEODS game has more detail and micromanagement but is more suited for PBEM.
I havent played GGWBtS in years but just fired it up after trouncing the AI in ACW. I prefer GG's map and I like the concept of theater commanders. Sometimes, its nice to worry about the big picture. With AGEODS ACW, I watch the big picture but Im also screwing around with independent divisions here and there. Put it this way, in GGWBtS, you are Grant post March 1864. In AGEODS game, you are every general from Grant on down.
So, basically, AGEODs ACW offers more detail but GG is certainly not lacking in that area....its just more macro. If you like strategic ACW play, you will like GGWBtS. I will tell you it is NOT as complex as Gary's other games like WiTP and WiTE. Thats a good thing.