Thanks for the question. I am forced to agree with Chimera
Seriously, it IS a good question, and deserves an answer. I can give you a few considerations when thinking about what game to get (if you can't buy them both).
1. Raging Tiger is JUST about a hypothetical future war in North Korea. It does cover a wide range of engagement and force types, from amphibious ops to tank fights to light infantry tactics. But it is JUST about Korea.
ATF gives a less detailed treatment of three different campaigns: The National Training Center at Fort Irwin, Fuld Gap, Iraq, and an NTC-Like mini-campaign in Death Valley, California.
2. If you want to run ATF scenarios in Raging Tiger, you must install ATF first, and then install Raging Tiger OVER it. The reverse might cause problems.
3. You can only run ATF scenario sin Raging Tiger IF YOU OWN BOTH GAMES. Of course, custom ATF scenarios will run in Raging Tiger, but if they are based on ATF maps, you will have to have ATF.
I hope all of that helps with your decision.