The D7800 has the ADSL but not supported by OpenWrt anyway, the pictures of D7800 make it look like a poor relation but listings on ebay have them looking similar.
Is there a preferred model from the two above or any tech reason?
I noticed the Broken NAND Chips/Blocks issue on the R7800 but the fix does not look too onerous.
If the ADSL was supported on the D7800 then it might be worth considering for future use.
a) chances for the xDSL modem to be supported in the future are near null.
b) older SOC (ipq8064 vs ipq8065), running at lower clock frequencies (2*1.4 GHz vs 2*1.7 GHz)
c) older wireless chipset (QCA9980 vs QCA9984)
d) half the RAM (256 MB vs 512 MB)
Choose the r7800 if you can, because the community support is bigger as this device has more ‚active‘ developers and users.
And the only feature (vdsl modem) which really makes a difference is not supported by openwrt.
(From a d7800 user which has long experience with this device)