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Having MAME run well was one of the main factors for me upgrading to the 7800.  I came from the old s5110 on which I had a great skelton rom installed and in my view transformed the device - the device as a whole didn't really have the oomph needed for some of the high end stuff and 0.139 MAME set was a no go (did handle the 0.375b set reasonably well).

I have been running the 7800 with webclaw's rom for some time now and it has been stable for me (there are more recent ones which are reported as excellent as well).  I have worked through most of the MAME stuff on the device and the games worth playing all run very well.  Haven't got round to the reicast stuff yet but all the other major emulators work great and have had recent upgrades to improve performance (PPSSPP).  I really don't think you will be disappointed with the performance of the device as a whole - as you know what to expect from JXD build quality which is a downside.

Well, I think the S7800B is a bit short of battery, and power-wise starts to be a little outdated.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...