the XR is more powerful then the X series in may ways becasue its just focused on sounds instead of having to worry bout a sequncer etc.... it has more expansion slots, more filters,etc... if you go to the site(rolland.com) it explains this in detail...
http://www.roland.com/products/en/Fantom-XR/index.html
You can look and compare on what each of the X series does
Yes there is stuff the X6 can do that the XR cant, and thats sequnce... the XR is purley for sounds....
im not trying to be an asshole but alot of the questions your asking can be found with a pretty simple search... Make sure you look before you ask, save the questions for the "real" problems