I'll start this review by admitting that I never played the Wii U version.
The last Mario game I played was New Super Mario Bros., back on the original Wii.
Coming from that game, I'd say that New Super Mario Bros. U Deluxe has several welcome improvements, the best of which is that you can choose your character, and that there are a couple Easier/Easiest characters available.
My six-year-old son loves everything Mario, so when he saw this he was super excited. I was a bit skeptical, because I was worried that it'd be too challenging for him, but I figured I wasn't much older than him when I cut my teeth on the meatgrinder that was the original NES Super Mario Bros, so I took a chance and ordered it.
So happy I did. It's been a ton of fun and a great bonding time. I absolutely recommend it if you've got younger gamers at home, who either want to play with you or their older brothers/sisters.
I played through the entire game -- every stage -- with my kid. I was Mario and he was Nabbit. Nabbit (from Rayman, maybe? He's a little bandit bunny thing) is the character the game considers "easiest" -- as far as I can tell, Nabbit can't run, but enemies also don't notice or hurt him, and things that would harm him (e.g., fireballs) only stun him. The only way Nabbit can die is if he falls into a pit (or lava).
That means that a younger kid (say, a 6-year-old) can play with someone who is older. I played the entire game with my young son, and together we beat the whole thing (there were a few levels on Super Star Road where he couldn't keep up, but those are extra challenge stages unlocked after you beat the game).
We had a ton of fun over probably 25 hours or so (we moved fairly slow, because my kid is young), and I'd say it was absolutely worth every penny.
That said, I have two very mild criticisms. First, it's a fun game, but not the best Mario game I've played. It didn't seem like there was much variety in the power-ups (Mushroom, Fire/Ice flowers, and Acorns -- although some others, like the penguin suit or the propeller head were also available, rarely). Difficulty wasn't as high as some others; for me as a dad playing with my kid, this was absolutely perfect, but for me alone -- a mid-30s guy who has been gaming all my life -- the biggest challenge came from trying not to accidentally bump into my kid and screw up a jump. (Four-player mode would be a nightmare, I'd think.)
Second, my kid was disappointed that Yoshi hardly made an appearance. Realistically, he was probably only present on three or four stages, and you couldn't take him with you after you reached the flagpole. Yoshi is a staple character of the Mario franchise now, and a favorite of kids, so in a game that's designed to accommodate younger gamers, you'd think they'd include those kids' favorite character.