MS Flight Sim is like X Plane. If you like one, you'll like the other.
SFG has a whole group of similar sims with different terrains, planes, and eras. They cover Vietnam, Israel, Europe, and WWI. Check out www.thirdwire.com to see them. Like SFG, they're all very easy to just jump into, fly, and fight.
LockOn is more complex with more to do in the cockpits and more accurate radar and avionics controls. An updated LOMAC 2.0 is due out soon.
If you like WWII, Il-2 is the place to go.
If you like WWI, the CFS3 mod OFF is great, and some people have seen past the shortcomings of the new Rise of Flight to enjoy that.
Falcon 4:Allied Force and DCS: Black Shark are the pinnacle of realism with a single craft modeled and TONS of switches and knobs to learn. An A-10 is in the works for the DCS series that will let you fly both together.