I personally compare my Wii U to my PS3, not to my PS4... it makes more sense IMHO.
The hardware power of the Wii U is much closer to the PS3 than to the PS4. Most multiplatform games look almost the same on Wii U and PS3 (some slightly better on Wii U, others slightly better on PS3). Some Wii U exclusives look better than most PS3 games, but some PS3 exclusives also look better than most Wii U games... it's a tie for now, but the Nintendo exclusives next year will probably tip the scale.
Both WiiU and PS3 are backwards compatible. My PS3 can play all retail PS1 games, many digital PS1 games, most retail PS2 games and many digital PS2 games. Many PS2 games are also available as upgraded HD versions for good prices... digital and physical.
My Wii U can only play retail Wii games and no GameCube games... I can't buy Wii games digitally, which is a bummer (especially these out-of-print-titles like Xenoblade Chonicles). Only very few GameCube or Wii games are available as upgraded HD version (yet).
Both systems (Wii U + PS3) have free multiplayer, neither of them needs an online subscription. And there are (naturally) many more games with online multiplayer out there for PS3 than for Wii U. The Wii U has a clear advantage in local multiplayer, which is great for families (if the parents or siblings or friends in the neighborhood are into Nintendo games), but it doesn't matter for many adult gamers (friends lost interest in gaming, don't live in the neighborhood anymore or have rarely time to come around to play video games).
And I have my doubts, that the next Nintendo home console will be backwards compatible to WiiU games... they will have the same problems when changing the architecture as Sony and Microsoft had with the switch to x86-APUs.
