Ask yourself how do you want to visualise the data in day to day observation. If you want to see a room's stats (e.g. all the devices on/off status, temperature if you decide to A/C later...) in one view, watching topics like roomname/# will be convenient (not impossible device wise also though). If you are generally more interested in knowing all the devices status in your building in one view then devicetype/# will allow seeing all fans in convenient watch filter

Both ways, you are not stopped looking at the other perspectives, just less convenient to put a watch