You can actually play Crysis using the updated graphics found in Warhead. Check out this guide.
Crysis IN WARHEAD ENGINE TWEAK - Notebook Forums and Laptop Discussion
I loved the KPA levels in Crysis, as said above cool sneaking around, more open world than Warhead. But the Aliens and last part of the game were kinda meh. Warhead was great action all the way through but not much opportunity for planning attacks etc. Both are very fun and I recommend both. But if you want just one... I'd get Crysis for the slightly more open world.