The Stewart FT has better detail, and better running characteristics, but, in the A B set, only the A is powered, and they are pricy. Bachmann sells the B as a separate so it is safe to assume that it is powered. I've only seen the F7, and it runs as well as the basic Athearn*, with about the same level of detail.
* I still consider Athearn the "best bang for the buck", an excellent "mid-range" product, and a great starter product. Not as much detail as their genesis line, and better running than most Trainset locos.
Pete