I was in the TAGB for 2 years and got my Blue Belt, I have now changed over to the ITF and just gained my red tag.
I have found 1 significant difference between the 2 types of taekwondo.
The TAGB say they teach traditional taekwondo, which means it has not changed over the years, but the ITF has progressed the art with the times.
The difference between the 2 is the ITF have introduced the SINE wave to produce more power with your punches, blocks and kicks, where as the TAGB still use the hip twist.
Here a couple of videos of General Choi Hong Hi explaining the SINE wave movement.