In MMA the kicking range is similar to the takedown range, with regard distance between fighters. Therefore, I would imagine punching is more desirable as it allows a better sprawl and takedown defense. However, in the clinch range again takedowns are possible (Greco/Judo) but punching isn't as effective as knee and elbow strikes.

I know that comment bears little significance, just thought I'd say it as I have to kill two mintues before I leave for training.