Also, since this thread is back up, the real answer is 'apples and oranges'. Their feature sets are totally different.
LOMAC has excellent graphics, a larger flyable plane set, and is certainly more accessible.
As far as simulation fidelity, well, F4 is a true study sim for the F-16. There's never been a game that simulates a fighter plane in better detail, and I wonder if there ever will be. That said, even AF is a significant step back in graphics, and unless you enjoy mastering hundreds of buttons and functions before being able to really get your value out of the campaign, then this is not a game for you.
As far as my own preferences, well, I guess it might be easy to tell - LOMAC I tried to get into 4 times already and still cannot; Falcon 4.0 is presently celebrating its 8th anniversary on my hard drive
