I guess i'm biased because i'm 27. There hasn't been too many 2D console Mario games I haven't owned or played extensively.
I think they are so close in quality it's hard to pick a winner. Which shouldn't be the case. With HD graphics (finally) and several generations to improve upon. Wii U's games should crush the NES and SNES.
I think part of it is things we as older gamers expect and are dissappointed by.
I really liked the parkour element of wall-kicks and triple jumps. I really liked the new powers, spinny mushrooms, acorns etc
I was really disappointed Yoshi was temporary power up, that is barely a power up. I was really disappointed some of the older powers didn't make a return (frog, cape, hammer bros etc)
If you were to play Super Mario World and then immediately play New Super Mario Bros Wii I think that SNES would hold up.
I'm in my 30's and my first game was SMB/Duck Hunt/WCTM. I've played all of the SMB/NSMB games, except for the DS/3DS. My opinion...
NSMB(U) both captured a lot of the great things that people were thrilled over when SMW came out. The ability to go back, hidden exits, more of a "choose your own adventure" type as opposed to the more structured SMB-SMB3 games.
I do love the newer ones. But I think one of the negatives is that, with the ability to go back and re-do levels as much as you want, some of the challenge is gone. Remember SMB3 when Tanooki Mario was rare? Same with P-Wings. You used it, and that was it. You can't go back and just replay that mini-level to get it again. You can't go back and redo those Hammer Bros. or that ship to get those items back. Even finish a level in SMB3 and be disappointed you didn't get enough coins for the white mushroom house? Can't go back and retry it.
I do like the flexibility of the newer games. I just don't like that it can be made a "just rush to the flag because I can re-do it later" game. But I think the series is great.