Crysis was alright, but got fairly... odd story-wise with the second one towards the end. Halo I think has been better overall for plot, and Halo 4 was the best of the series I've played so far.
If I could somehow make a game that has the plot and setting of Halo (I like the more sci-fi level Halo's on) but combine it with the gameplay and mechanics that were in Crysis 2 (being able to evolve your suit and play the game in different ways) then I think I would be a happy man.
Multiplayer... I don't play multiplayer, so I don't know how they differ. I do know that Halo 4 has a loadout option thingy like in some of the Call of Duty games.