Do you have the money for a 1d X and get by with 18mp, get it! It will be better in every way. If you don't know if you need a 1d and you choose between a 1d and 5d based on resolution, yeah, that answer it for me, you don't need a 1d.
I use both 5d2 and 1d4 and for things that need to be perfect I use the 1d, if I'm doing portraits and less stress situations, I use the 5d because of the FF sensor (although the lack of useful af-points makes it much less able to shoot like I want to, wide open at 1,2 and 1,4), the only thing the 1d4 lacks. So I'm first in line for the 1d X. Giving me the perfect camera, but the FF sensor is just a very welcome bonus. It's the AF-system that is highest on my list when choosing a camera.
And be prepared, once you go 1d, you'll never buy another camera than a 1d.....