well ive heard both x's and r's many times. the x's have nice sq, but I, myself, prefer the R's over the X's.. the output and sq that the R's produce is great. alpine makes some top notch subs.
the type X is a high excursion SQ sub. It's equivalent to the RE XXX and JL W7 in terms of design. While it gets plenty loud, it's mainly designed to be a SQ sub.
The Type R is an affordable all around daily driver workhorse sub. It does very well in a variety of enclosure alignments and doesn't require gobs of power.