With the RD8 and RD9, you are getting a much more limited set of internal sounds. I love the sound of an 808 and the RD8 sounds awesome, but I still need to process individual sounds externally. The TR8S has extremely reasonable approximations of all the classic Roland drum machines as well as sample playback capability.
I bought the RD8 mainly as a performance tool, to use the 3 trigger outs to interface with my modular. I have some decent 909 sounds in the Pique and FX Aid XL module, as well as the Blck_Noir, Crucible, and Basimilus Iteritas Alter. I don't really need the RD9. I can't say I'm unhappy with anything the RD8 offers at the price.