they're both absolutely essential in mma.. kicks and knees can change fights, but i honestly think boxing is more important between the two, as long as you have a good understanding of clinches, low kicks and checking low kicks, where your liver is, etc. i think you'll benefit more from improving your hand speed and punching power
however, never confuse boxing and mma, they're not even close to the same sport, think of it as you're training one sport to get better at another, you can't train boxing expecting to compete in mma, it just doesn't make sense.. but boxing is a great stand up base and as you progress you'll notice the obvious differences between boxing someone and fighting someone that's wearing 6 ounce gloves and is allowed to knee/elbow/kick you in the face