If the kickboxing instructor is you kung fu instructor, then I would go to the Muay Thai to get some fresh new stuff. Plus, doing MT will be going to the source. You will get plenty of cardio while you are there. And get some really good clinch training. Do the MT for a while and see what you think. You won't be disappointed.
Plus uni martial arts classes are ze cheap. I could train bjj under a purple belt at one of the campus' here for $100 per 6 months, peanuts.
Get the best of both worlds, train the MT and enjoy having the extra students in your kung fu class to spar with while testing out new stuff.
I can't wait for the mt classes,have been wanting to do it for ages but the nearest class was too far away really, so this is on my doorstep. Think a fair few of my mma class will be going there too.
I would quit the kung fu but it's on the same night I train mma, so I like the stamina it's building in me ,it's fun and I get some good sparring training there (believe it or not!)