i had 2 type-x's and for grins i bought 2 type-r's from bb, put them into the same box, hooked up to the same amp, and there wasn't a noticeable increase in volume, but there WAS a slight descrease in transient response, but nothing drastic
type-x speakers have better power handling, and are better geared for low-end sound to accompany a full upgraded sound system
the type-r's don't handle quite as much power, but they have better output in the higher frequency range, and are better suited for high-tuned ported enclosures
there's honestly not a HUGE difference in reponse from the two speakers, it mostly depends on the music you listen to, the rest of your system, and what you intend to do with the speakers
to enhance an already upgraded system with true sub-bass, or if you're planning on a sealed box, go with the x
if you're just trying to add bass, trying to compete, or are using a prefab, get the type-r's