i use an r5 and ive heard much about the r8. to my understanding the r8 is exactly the same sequencing as the r5, but with an expandable soundset. in other words you can buy drum kits for the r8.
if you are using the machine for midi triggering and arent interested in the sounds of the drum machine, i'd just go with the r5. thats what i have, and believe me it is a BEAST for drum sequencing. you can even sequence drum-n-bass type sequences. tiiiight sequencing!!!!