WT-7 gives more range but these cameras have built in wifi capability to transfer images to PC.
I wouldn't hold my breath on the (probable) transfer speed of the built in Wifi system.
While they claim AC wifi built in, it's not going to be the higher specced 3x3 mode, which gives decent transfer speed.
The WT-7 may have the 2x2 mode for them to claim the 866Mbps transfer rate.
So using the (camera)Wifi to transfer full RAW images(probably 80+Mb file sizes) will take a while in a mobile situation. If you backup jpgs, it may work quickly.
My personal feelings of the dual card situation is indifference. D800E has dual card slots, I only use the CF!
I had an SD card in there for capturing video for a while, never used it, pulled the SD card for another device, never replaced it.
I'm thinking that it may have been a deliberate choice to allow them an update path for the future. (ie. in the same way they updated the D300 -> D300s) or an upgrade model (possible Z8 or something like that).