Just as always, it's a personal choice and I respect anyone's choice, but I have my own reasons to have a clear winner here. And for me that's an easy pick... Super Mario Bros 3 was the real best among them two.
I find it comprehensive to new retro-players to go to SMW, but I started playing videogames with a NES instead of a SNES. Think about the slight quality differences between SMB3 and SMW and now do the same with SMB3 and SMB2 USA or SMB The Lost Levels... SMB3 was the real mind-blowing game at its time, so mind-blowing that SMW did not really surprised anyone that played SMB3 before. It's something similar to Zelda BOTW and Zelda TOTK... TOTK expanded and polished it all from BOTW, but BOTW is considered the real best Zelda game of all time.
Big questions apart, I hate Mario's desing from SMW when the cap turns up when jumping, almost falling from his head (exactly the same with Super Mario Wonder). Please Nintendo, fix Mario's cap for the sake of god...

Besides that, when talking about levels designed in Super Mario Maker 2 with the SMB3 engine, I find them much prettier than the ones done with the SMW engine.
And of course there is one of the best remakes of all time in the contest, which is Super Mario All Stars. The amazing re-work done with this quadrilogy is a piece of history of the pixel art. I'd still pick SMB3 from All Stars over SMW.
Maximum respect to SMW (A.K.A. Super Mario Bros 4 in Japan), 4th place in my list of best 2D Mario retro-games, but that's my humble opinion.