The SF series has a gentler learning curve than LOMAC, it is also easier to get quick fun from it than from LOMAC.
LOMAC is more centered on 90's/00's while SF is more centered around 50's to early 80's.
If eyecandy is your thing, then LOMAC can't be beat.
If missile lobbing is your thing, then LOMAC is probably the best choice.
If the only real kill is a gun kill, then SF is your cup of tea.
If you like to spend as much time creating as flying, then SF will delight you.
If you want to play online, then LOMAC is your only option (or SF1 games).
As for which one to get if you decide to go for SF, it's all up to you.
SF2 is probably the lesser choice, it is fictional, with an uninspired terrain, however it has ressources the other games have not AFAIR.
SF2V has terrain and a planeset for the Vietnam war, and the 3rd party Air & Ground War Expansion Pack takes it to a whole other level.
SF2E is for an hypothetical WW3 in Germany, with possible dates between the 60's and 80's, once again you can make it even better with a 3rd party effort, NATO Fighters.
SF2 uses an "open" structure, making it easy to create and install mods, some have installers, but for most it consists of copying directories, most mods these days are pretty straightforward to install and come with readmes, and the community is always here to lend a hand.
Mods range from skins to complete changes (with new maps, new planes, cockpits, skins, weapons, tanks).
You can browse the Downloads section to get an idea of what the community offers.