FYI: too many owners of R9000 are dissatisfied by stability of its Wi-Fi (permanent dropouts). My version tries to fix it. But seems it is some kind of chronicle illness. I cannot fix everything because of lacking source codes and NG silent refusal to get some kind of cooperation with me. Well, problems with Wi-Fi stability is also reported for other NG routers with latest stock FW so it is something like a rule...
According to others my latest FW for R9000 is free of this problem. R9000 is my main router and no dropouts for e.g. 10 days.
Anyway. Being in your shoes I would choose R9000 (R7800 clone vs R9000 for the same price). But if I would need to spend my own money I'd choose R7800 (much cheaper and stable).
Voxel.