I'm assuming for the 3ds you're referring to 3d Land, not 3d World and I disagree.
I bring this up because 3d Land is a game I initially believed would be average, but turned out to be one of the best 3d platformers I've ever played and it slightly edges over 3d World for me (and yes I've played through 3d World with 4 players - at its best).
The idea that you need the 3d feature on to play 3d Land is false. Yes there are segments which are made easier by the 3d effect, however, these are few and far between and it is certainly not require for 95% of the game. (As a gamer who played 3ds extensively, I never left the 3d effect on for more than 30 mins on the lowest setting and I personally wouldn't recommend anyone to do this.) If anything 3d World, which boasted more complex levels, I could've benefited from 3d glasses on specific levels where you're on a fast-moving platform and it can be difficult to judge the depth perception when you need to make certain jumps (similar to Axo-Battle Toads, but not as brutal in difficulty).
Content-wise, 3dL had 8 worlds, special stages, more bite-size levels which were conducive to a 1 player experience and a much more satisfying final boss battle. 3dW was more complex, more graphically nice, had the cat mechanic which was interesting but was only great with 4 people, had too many racing/ on-rails segments, only had 7 worlds and a lame final boss battle which killed the rest of the game's momentum imho. It also had the golden 'cheat' Tanooki leaf, which there should've been an option to turn-off, or simply give the player the P-Wing on rare occasions like SMB3 did.
For me personally, SMB3 as a kid, was my favourite Mario game of all time (technically All-Stars, but if I had to choose one game it would be the All-Stars version of SMB3). This basically remained unchanged, although New SMB was a rival contender for some time, but 3DL took it and for the few 3D Mario's I've completed, it still holds the torch which is yet to be passed.
Yeah, I agree that 3D Land doesn't need the 3D effect to be enjoyed, but early on I read the contrary from several people online, giving me a false impression of the game. I regret ever listening to them because I LOVED 3D Land after finally playing it.
As far as which of the two 3D games I prefer, I do slightly prefer 3D World simply because I can use a proper gamepad and play on a big screen, but purely from a game design standpoint, I feel like they are really close to each other.
As for SMB3, yes, I am part of the Mario 3 = GOAT Mario gang. Below is a picture of one of the walls in my game development office!
(https://i.imgur.com/7p5N5WZ.png)