masters choice

when you pick which hideout you want is it purely cosmetic?

and then you can level all the masters up?

You can have any master in any hideout.
Simply dismiss the ones you don't want and hire the ones you want.
Having a master in your hideout gives you the possibility of doing his dailys.
Those dailys work just as if you encounter them in the wild, but give a bonus to reputation gained.

Which hideout you choose is not purely cosmetic, as the amount of masters you can have in a given hideout depends on the level of that master that hideout is from. However, should two masters have the same level, then there is only a cosmetic difference.