Hi!
Thanks for the question.
There is no online manual for AATF or BGC, but there is a downloadable demo for AATF available here:
http://www.prosimco.com/free_demo.htm
It includes an interactive tutorial that will show you the game mechanics.
As for which game is best, that really depends on your tastes. BCG is an armored simulation based on British equipment, while AATF simulates air assault and light infantry tactics. AATF has a lot more missions, while BCG is a mini-game with just a couple of scenarios (at a lower price-point if you just want to check out the game before you make a bigger commitment).
One other thing...both game systems are fully compatible with the ATF series of games, so you can install the game over any of those games and play the older scenarios in the newer engine, with all of the additional features (like the more complex damage system).