Hey Shorty,
Here is my opinion. There are a couple of differences.
BCT is Brigade level - ATF is Company to Battalion level.
ATF took the best of BCT and built upon it while changing the scale.
One of the best features of ATF is the ability to install the next game in the series in the same folder and utilize the next iteration of the engine with the scenarios from ATF. You can check this thread under AATF (Air Assault Task Force)
http://www.shrapnelcommunity.com/thr...b=5&o=&fpart=1
This will allow you to utilize the new AATF GUI with the four previous games in the series.
You dont have to overwrite, and you can even do both. You can install each game to its own folder and another install to a "combined" folder and see which you prefer.