which should I choose? JXD S7800B or S5800

[Made in China]

As Made in China, I can say that not all of the products are bad.
You should, however, steer clear away from JXD. Lack of support for their recent devices (S7800 and the S5800) plus them having closed source kernel, meaning that there are some problems with them that you simply cannot fix.
Plus, all later models S7800B have the capped rk3188-t SoC, so you're being screwed.
Go for some of the GPD devices if you're into that kind of thing, they're more community friendly and cost less.

I personally wait for the rumored nVidia Shield 2 with the Tegra K1 SoC or for a good 5'' phone with small controls (look up the design on the Much W1 - only without shitty firmware choices). Either of those things is going to revolutionize the market - because both unlock possibilities that were unachievable for Android before (classic gaming on the go and high-level gaming on a portable machine).