Wow 18 posts and no Chris Rock references? 
Just get a gun.
I was asking about this on here a while ago. Unfortunately I’m a busy/lazy bastard and ultimately did nothing. However, the impression I got was that Krav Maga was more “here’s how to dead someone else when they try to dead you” and Muay Tai was more of an exercise/sport.
Granted they’ll both give you a workout and they’ll both teach you self defense, but that was what the difference seemed to be.
Here, Micah & friends gave me some good info:
http://nyspeed.com/forums/showthread.php?t=33897&highlight=martial+arts