Well, I think the S7800B is a bit short of battery, and power-wise starts to be a little outdated. Still, it is sturdy and the build quality is very good (to be a JXD product), controls are OK. The downside is that flashing firmware is a pain in the ass until you short all the issues and that the existing ROMS are not as good as skelton's.
I got a Q9 the other days, and to be honest I think I still prefer the S7800B. Build quality is much worse, it seems that it is going to break, as is the screen (to be IPS, mind). The dpad is very good but does not seem made for intensive hadoken gaming, seems fragile, the S7800B gets the job done and you can trust it won't break. The analogs are pretty bad, even worse than the S7800B's
If you only want retro, I would probably go for a Q89. It is much cheaper than the Q9 and the S7800B but has the improved battery and wifi of the Q9. At that price is a good deal.
Still, in my opinion the best consoles now are 5": the G5A, but you have to be lucky with the DPAD. Many have issues, including mine. And the S5800, more expensive and less powerful, but more balanced (better build quality and all controls work well, plus the battery is a long distance runner). Of course, if you don't mind paying 3 times more, the Shield and the Much i5 are even better...