Well in that case I suggest 720p cause 1080i causes the game to look a bit weird on my tv (just tested it with 3d world) But idk, that might be the settings

I'd take 1080p and 30fps over 720p and 60fps for almost every single game other than the most demanding shooters. 

and racers, and fighting games

What is the actual resolution of the tv panel?
If it's 1280x720 native, then 720p is your best option.
If it's 1366x768 you might get some more detail with 1080i. (1920x540 alternating lines) The difference will be minor anyway.

It also depends on what your tv does, de-interlacing and scaling produces extra lag. Easy to check while holding the WiiU game pad up next to the tv. 720p is probably faster.

Racers and fighting games, at least for me (and I know everyone has different preferences on this) are perfectly playable iwith 30fps, assuming there is little to no drop off. The only reason why 60fps is so important in shooters is because of the precise, on-the-dot aiming needed and the need to compensate for any server lag in multiplayer.

Well, like that article says, it has a lot to do with how good the particular TV's video processing is.  I would suggest trying it on both settings and see what looks better to you:

"the most telltale sign that a processor is not doing a good job is to look for any jagged edges on objects in the image. This will be more noticeable on incoming 1080i signals as the TVs processor only has to scale the resolution up to 1080p or down to 720p (or 768p), but also has to perform a task called "deinterlacing". Deinterlacing requires that the TV's processor combine the odd and even lines or pixel rows of the incoming interlaced 1080i image into a single progressive image to be displayed at least every 60th of a second. Some processors do this very well, and some don't."

From my personal experience it kinda comes down to if you were only viewing still photos, then 1080i would produce more detail, but with a picture that's constantly in motion (such as a show or a game) 720p is superior.  Although there are theoretically less pixels in the 720p image, the entire picture being rendered in a single pass (progressively) looks better and smoother in motion.

Try to play Mario Kart @30 fps

I do, and it's perfectly fine when there is no lag. 30fps is perfectly fine when there is no drop off in speed, and the only problems I have are when the fps drops below 30, which is when it is hard to play. A steady 30fps is perfectly playable.

OT: 1080i if you have a TV more than 1280 x 720, if you have a 1280 x 720 TV stick with 720p.

720p. I've tried watching quite a few shows with a 1080i resolution and it was absolutely terrible. The enire time all I could see was the jagged edges.

Except when the game is 30fps and the tv de-interlaces correctly it's as stable as viewing still photos. Every frame gets send in 2 halves, while it's always sending at 1080i60. For 60fps games you might see this effect

Either a bit blurry where correctly de-interlaced, or two frames weaved together (not de-interlaced at all)