I played T:A a bit today, I have been playing firefall a lot so it messed with my feel for the game. What I really noticed is that in firefall you don't have skiing or the top speeds that even T:A has, but the physics of T:A are so fucked that firefall seems faster, even in the heaviest class in firefall you don't feel like your a weighted down as much as a light in T:A.
The jet packs in firefall have less energy so you can't use them as long and you can't go very high but when you use them you move up at a respectable speed and the momentum causes you to drift upwards after the jets stop because of the speed.
Going between the 2 games it really makes me wonder how hirez decided on the physics they did, wondering if they decided that the armor is heavy so the jetpacks struggle to lift a character. Firefall is in very early beta and is probably going to be in beta for at least another 9 months, yet they already figured out a decent feel for jetpacks.
edit: did some testing, wasn't getting much drift in firefall at the top of the jetpack jump, still feels better than T:A though.