Definately boxing. My instructor was a kicker with few peers, yet in his bar-bouncing days he ended almost every confrontation that escalated to a fight with a standing submission hold (Hapkido/Judo) or punching. Boxing is more adaptable and applicable to a greater variety of situations (hard to use kicks to great effect in a crowded situation, such as a bar or a party, which is where I've seen most fights go down). Plus, I think kicks (even if done well) must be performed at a greater risk of getting countered badly by a skilled opponent than punching.
And I personally think-- based on observation and personal experience-- that if a boxer and a kicker of comparable skill go to head-to-head, my money is on the boxer.
Don't get me wrong...Muay Thai is a spartan, brutally effective striking style. But if I had to choose...