I have beaten both of these games numerous times, and I have always enjoyed Super Mario World more than Super Mario Bros. 3, though the latter is undoubtedly a great game too. Super Mario Bros. The levels had more variety to them, and the secret levels were great. I loved the red colored levels where I would have to find the key and lock, or the unforgettable secret finish behing the normal finish. I loved the overworld design and layout, seeing that it was huge and interconnected. Super Mario Bros. 3 had interesting themed worlds (Giant World!), but Super Mario World's map was more interesting to me. Super Mario World also seemed to expand and keep expanding, as it had a Star World, and a Special World, which gave players extra difficulty in the levels. Some of those Special World Levels are TOUGH (Tubular, I'm looking at you). Both games had good final Bowser fights, but I liked Super Mario World's three-staged Bowser battle more.
So all in all, they're both among the best Mario games, but Super Mario World just had a bit more than Super Mario Bros. 3 that makes me love that game.