I've owned an R-8M for 20+ years. It was my work horse drum module during the 90s.
The "stock" sounds on the R-8 and R-8m are quality, but typical late 80s and early 90s acoustic drum sounds. The main thing about the R-8 Human Rhythm Composer was being able to adjust the nuance of the drum sounds to make it sound like an actual drummer not hitting the drums or cymbals in the same spot with each hit. So if you want a machine that sounds like a real drummer without tedious programming, the R-8 is the machine for you.
The R-8 has one PCM card slot (the R-8M has 3) for adding your choice from 11 different PCM card with additional sounds like Jazz Brush, Ethnic percussion, Sound FX, Mallet percussion AND Electronic (TR-808 & Synth Percussion) and Dance (TR-909 & CR-78). The R-8mkII includes all the sounds from the R-8 plus select sounds from the 11 PCM cards.
Now if you want a machine that does a damn-near-accurate reproduction of the TR-808 & TR-909 sounds, then the TR-8 is the machine you want.
One drag about the TR-8 is that you can only choose one sound per instrument type per kit. For example: for bass drum, you can only choose 808 kick or 909 kick, for snare: 808 snare or 909 snare, and so forth. So if and when Roland adds TR-707 sounds to the TR-8, then you'll have to choose one of 3 different sounds (707, 808 or 909) for each drum type.
That's not a big deal for many, but I've made drum beats that have multiple kicks and snares. But at a price of $500 or less, you can afford to buy 2 TR-8s and dedicate one as a TR-808 and the other as TR-909 if you want.
A third option for 808/909 sounds would be a Novation DrumStation. The DrumStation gives you access to all 808 & 909 sounds simultaneously and has more individual outs, but lacks a sequencer - so you'll need to create your patterns on your DAW or hardware sequencer.