[天の龍]

Actually - in terms of sheer numbers - pop'n is more popular than IIDX. Stick around an arcade long enough and you'll see more people play pop'n than IIDX.

However - IIDX seems to have a more dedicated fanbase than pop'n does. More of the same people will frequent the same machine more often than a typical pop'n machine. I personally know a dozen friends that I play with while I'm here, while only a few from pop'n - even just from lazing around.

This is strictly from a local perspective though. I've not much information according to national statistics.