That all depends on your preferences and the size of the rain shower head that you plan to install.
A standard head may seem to be the best as the installation of a new head is as simple as taking one off and putting the other on. Be careful though, as standard arms typically do not extend too far from the wall. If you’re putting in a larger shower head (for example, a 10” square) then you may have problems getting it to fit.
Extended arms can be purchased that will certainly eliminate this problem. Better yet, you can get arms that are not only longer but can be adjusted as well. That way you can position your new rain shower head exactly where you need it to be (I’m 6” tall, and having that adjustability can make a big difference).