It's a matter of personal preference. No right or wrong answer.With Main Control on the outside you may have to install a temperature sensor inside the shower.Also, having the outside will allow you to start up the steam shower without opening the door and entering.The downside is that while you are inside the shower you will not be able to access the steam control without opening the door.The choice is yours.....Just for reference, I have my control on the inside and I'm happy I do since sometimes I adjust temperature or even just for simply turning it off.