1080p always.

Personally, I would choose 1080p Wii U visuals, but the general better choice would probably be better graphics at 720 p... : /

Better details 1080p.

Well. Not entirely accurate.
My Android tablet hooks up to the TV and can emulate a dozen differently platforms... And even has physical controls.

Let's just say... I have never bothered to hook it up to the TV, there is no point.

720p

Smash Bros was limited by the Wii U's lack of memory bandwidth, so it used very simple lighting.

The NX, if Nintendo doesn't seriously goof off with power again, might be able to do proper lighting at 1080p.  

TV mode　1080p
Handheld mode 720p

They're gonna need some bandwidth for 1080P at home I think.

50GB/sec is quite good for portable mode, but if they want to do the whole "it goes to 1080P with the home dock", they probably could use a 192-bit or 256-bit memory bus.

The 3DS already has a 128-bit bus, which kinda surprised me, so Nintendo going higher would make some sense for NX.

It also probably combined with Nvidia's tiling approach likely means Nintendo won't need eDRAM, which increases cost, heat, takes up a ton of die-space, and creates yield problems.

Even if NX has the juice to do Mario Kart 8/Captain Toad graphics at 1080p, the fact remains that rendering in 1080p means drawing twice as many pixels as 720p, which means half as much fillrate for things like shaders. I'd much rather that power be spent on x2 better shaders, effects, etc than the (in my opinion) rather negligible improvement that 1080p offers.

They won't go for 192-bit or 256-bit. Power Requirements would be far to high.
The higher the bus width, generally the more intricate the memory controller and memory layout and PCB layers become.

The 3DS could get away with it because it's RAM was clocked fairly conservately (200mhz) and it used FCRAM.
I calculated the speed based on the Bandwidth * Bus width rate. I.E. 200Mhz * 128 / 8 = 3.2Gb/s.

To put it in perspective... PC Graphics cards had Bandwidth equivalent to that 20 years ago. :P

eDRAM if used correctly can also reduce power consumption as Intel has shown us.