I haven't studied Kung Fu yet, but I hear it can be really effective when you use it to it's fullest.
For Muay Thai, I've seen the real damage it can do. So it's really hard to say because they're both really good styles. The thing I notice about Muay Thai nowadays is that it's simple yet extremely effective.
For now, I'll have to go with Muay Thai.
Yes, the simple arts are always the best. Muay thai is very effective for that reason. Muay thai also is great cos of its power. You learn a few basic moves and practice them over and over on a bag and pads, building up tremendous power.
Learning muay thai also gives you the upper hand in mentality too since that is probably the most important thing in a fighter. Muay thai guys are trained to destroy the opponent. No nonsense.
So once you learn it, your fighitng spirit is enhanced greatly
Other arts like kung fu are great too and if you train hard in it, you can defend yourself pretty well. Overall, learning any art will make you stronger and fitter and those alone will give you the advantage.
Although kung fu is great, a lot of it focus on forms and spirituality so make sure you are aware of whats effective in fights and whats not.