The main difference is FC3 is part of DCS World and gets all the updates and benefits of that. So there are more AI planes in DCS World (UH-1s, P-51s, A-10Cs), you can fly MP with other people, and the upcoming DX11 renderer will improve performance and appearance.
Oh, and the A-10A and F-15C are getting AFMs, so they'll fly better.
There are also periodic sales. The last was in December, the price was like 50% off at that time IIRC. If you can't swing the full price, wait for that, although it's hard to say how long that wait will be.
I do think it's worth it, though, as FC2 has some serious deficiencies in AA missiles that are getting fixed in DCS World. No more patches for FC2.