Wii will most surely be closer to the 720 and PS4 than the Wii is to the current 360 and PS3.
The problem with the Wii is it's ~DX7 GPU didn't have programmable shaders and it didn't have a multi-core CPU which made it extremely hard to scale games down to the system. Next gen though, all the systems will have multi-core CPUs and it'll more than likely come down to WiiU having a DX10.1 GPU while the PS4/720 will end up with a DX11 GPU. We'll have to wait until all the systems are out to see how big of a gap there is between the WiiU and the 720/PS4.
So really in the end, the Wii U should be able to receive better port support. By mid generation (next gen) games running on PS4 and 720 boxes at 1080p and 60FPS can get downscaled to 720p@30fps for wiiu. How good the support is depends on how good a job Nintendo does catering third parties and how well Wii U sells before PS4/720 are released and after.
The gap next gen will be more similar to PS2-GC-Xbox than Wii-360-PS3.
I have no doubt Wii U will get next gen ports since the industry has become so multiplat-dependent. What i'm worried about is how well will devs take advantage of the Wii U tablet. That thing has so much damn potential for gaming it would be an outright shame if it isn't taken as much advantage of as possible.
I'm optimistic that it'll end well though because the tablet isn't a completely new controller like Wii-remote was. It's a familiar dual analog with a screen and other enhancements.
sorry but wiiu runs on 1080p not 720