I've used xmax 10's and lots of different rsd sub setups. My favorite out of what I've used has been the rsd 12. It's a fantastic sub. Like Stipud mentioned, the low frequency response of the rsd is something to experience. Of course, I've still got some xmax 15's that I have yet to use and I'm really looking forward to the opportunity. I think that the larger displacement of the 15 along with the typical xmax snap will make a nice overall compromise.
I guess it really boils down to the type of music you listen to. Might give that RSD a try soon. If nothing else, my wifes car needs something to go along with her Exile comps.