Well, Wii U will never have "next-gen" graphics, simply put.
Although I think Monolith will make the best-looking game.
"Next Gen Graphics" in the context in which you used the term, is a misnomer, a self fulfilling prophecy.
All of the "next gen" systems (8th generation, read 'em and weep) will be capable of handling the same graphics engines, the same rendering techniques, and at pretty much the same level of fidelity.
Also, Wii U, being a next gen console, will define what "next gen" graphics are for the years to come. All of the "next gen" consoles will to some degree. Wii, grossly outpowered by its generational counterparts, still performed some "next gen" techniques better than both when it was released, namely tessellation.
Next gen isn't defined by FLOPS, RAM, GHz or what a particular game looks like. It is defined by the time of release. Now, we all know the PS4 and 720 to be more powerful than the Wii U, can't argue there. There have always been more powerful and less powerful systems in every generation. The difference will be very minute, and early on in the gen the disparity will be largely unused, and later, unnoticed.
I get the confusion, Nintendo themselves claimed they could "skip a generation" because they went past 32bit graphics and straight to 64bit, but that didn't exactly work out for them. Similarly, Sony believed PS3 would skip a generation and be sustainable for 10 years, based on the power profile of the console (behind the scenes based on the ridiculous losses they were taking on the hardware and their inability to forsee releasing another console in less than a decade) but that didn't work out for them either.
Graphically the 360 and the PS3 are drastically outperformed by their PC counterparts, but the graphics on those systems are "good enough" representations to continue to wow people. Wii U is considerably more powerful than those systems, with much more advanced features, using modern design principles. There won't be the fidelity gulf we were witness to between the HD twins and the Wii, it will be very close, very very close. The law of diminishing returns is a hard one to break, and PC's have been feeling the burn for quite some time. Simply adding more of the same doesn't necessarily cut it anymore, more polygons will largely go unnoticed. All three of the new consoles have support for some very nifty rendering techniques, and all will come with plenty of memory to give creators free reign to make large, immersive game worlds.