When choosing between LOMAC and SF2, there are few things to keep in mind.
Graphics:
SF2 uses DX10 shaders, LOMAC, even with the Flaminf Cliffs 2.0 addon, still uses older DX9.0c graphics. Now, LOMAC can still make the environment look better, but keep that in mind.
The terrains are markedly different. LOMAC's is much, MUCH, MUCH, more detailed, even with SF2's settings cranked all the way up.
Gameplay:
LOMAC only models a few aircraft, but they are modeled to a much greater degree of realism than SF2. Now, the realism can be scaled back to any skill level, but just keep in mind that SF2 was meant to be more approachable...easier.
Mods:
True there are mods available for them both, not as many for LOMAC, especially since the release of Flaming Cliffs 2.0, but you have to keep the quality in mind. SF2's stock aircraft reach a level of detail that third-party aircraft don't generally match. TK's models are not just pretty good, they're VERY good, so they raise the bar quite high. There are FAR more stock aircraft to fly in SF2, and an impossibly large number of mods you can download that allow you to fly anything nanywhere, but they may not have the level of realism you want or look the best.
pfunk