In my opinion Warhead > Crysis.
The environments are just as gorgeous, but to me the lighting and HDR effects as well as the volumetric fog and particles were even more impressive. You get to see a bit more of the transition areas between ice and jungle here and it is just as amazing as ever.
One of the big improvements to me was vehicle combat, the 3rd person view and aiming has been significantly improved in terms of feel, making it much easier to control the vehicles. Also there are more varied weapon loadouts this time around including the standard 50cal there are HE rounds launchers (my personal favorite), miniguns, etc.
I enjoyed the mission structure and pacing in Warhead more so than I did in Crysis. Cyrisis at times felt a little too open ended in approaching bases and usually I had to retry half a dozen times to get it it right. Warhead makes it a bit more straightforward and there is also less running around from A to B. The vehicle combat and ground missions are also mixed in well and you get to see a wide variety of environments. The large set peace battles are more entertaining as well since they are often just as interesting as Crysis, but it doesn't take half an hour to bring the bosses down.
My two gripes are that in some cases the weapons feel a bit too powerful for the situation (the final boss battle is amazing, but somehow feels anticlimactic with the weaponry you are given). And that the game ends too quickly. It only took me about 5-6 hours to complete on the first try. If it had been as long as Crysis I think it would have deserved a 10, but I will settle on a 9 since it is so short.
My advice, pick up the maximum pack and get both games, even though Warhead is awesome I can't justify the price for only 5 hours of play.