I have seen several examples of people migrating from phpBB to myBB, but never the other way around.Just a matter of preference really, phpBB3 is my choice but probably that's because I've invested quite a bit of time working on mods for it.
I do like its somewhat cleaner code compared to myBB though.
Just because more people use certain software dose not make it better (i.e. Windows vs. Linux/FreeBSD) and there are very few mods available for phpBB3.i think phpbb is better because more people use it and also more and better mods are for it