If you've got the choice, try and find a boxing gym, but if its the choice between an mma gym or training yourself, the mma gym wins every time as you'll still get the conditioning etc. What i would say from my own experience is that i went to train mma at the same time as i was training boxing, and the bloke taking the kickboxing area of the sessions was an ex muay thai champ and so was very set in his ways. Thge footwork in particular was very flat and he taught you to step rather than push off with the back leg when moving forwards etc. The stance was also a bit more square on and the hands were more palms facing forwards than you would expect with traditional western boxing, also the style was totally different, fighting in short burts rather than keeping on the jab etc. This didnt fit at all with my boxing and so i left to concentrate more on that.

Basically, if you was get training from a coach specialising in boxing then go for it, if you are going to have to train with the kickboxers and adapt their style to that of boxing then id personally stay away.

Hope that made sense and was a bit of help.