Price-wise on eBay, the TD-8 is generally as expensive as a TD-10. You can pick up either one in the $450 range. You can often pick up TDW-1 expanded TD-10 in the $550 range.
I'd vote to forget about the 8 and get the 10. If you can pop the extra $100, the expanded TD-10 gets you the enhanced V-Cymbal control and some other enhancements.
I've been extensively researching modules myself as I'm looking to upgrade from a TD-3. I'm personally waiting for the new TD-11, TD-15, TD-30 modules to be widely available to drive down the cost of pre-owned TD-12s and TD-20s. The expanded TD-10 is one hell of a module, though.
E Kit TD-10x (VEX Classic XP), PD-125 snare, (2) PD-100 toms, PD-120 floor tom, KD-85 kick, CY-15R ride, CY-12C/R crash, (2) CY-8 crash, CY-5 & FD-8 hi hat, MDS-9 rack, DW 5000 pedal, PM-30 amp.
A Kit Pearl Export 5pc (10"/12"/14" toms, 20" bd), 14" 1964 Ludwig Supraphonic snare, Zildjian Avedis 14" Hats, 14" K-Custom/16" Bronze/18" A-Custom crashes, 10" A-Custom splash, 20" Bronze ride, DW5000 pedal