This is the hardest question I have ever been asked.
Seriously. I'll talk about cons then pros of both games, then my decision.
SMB3 had awesome power ups, diverse worlds, and the ability to store items for later use. That gave it a bit more of an epic feeling. Toad houses, military vehicles, Kuribo's shoe, and more. Not to mention the warp whistles are some of the most cleverly hidden items ever. I didn't find the Desert Land one until I got it for GBA.
Super Mario World was great as well. It was far less linear than SMB3, and the secret exits were fun to find. The difficulty is a step up from SMB3(The fortresses and Special Levels were a pain) and Yoshi was awesome. Finding every little secret was fun. Soda Lake I never knew about until my cousin showed me a few years ago. It was a blast to discover it for the first time. Not to mention the cool little reward for all 96 exits...
SMB3 had its fair share of flaws, though. Many levels were either too easy or too hard in my opinion. Not to mention there really aren't any secret levels, beside the fact you can skip some without Warp Whistles. Many power ups didn't live up to their full potential. I felt the Frog Suit could have opened up many more secrets, and fire could open up more secrets in Ice Land. Kuribo's shoe is something I wished could have been used in more than 1 level.
Super Mario World also had several problems. One, the cape made many levels WAY too easy. If you had good handling, you would never stop flying. There is also a great lack of power ups compared to SMB3. SMB3 had-
-mushroom
-fire flower
-raccoon leaf
-starman
-kuribo's shoe(although limited)
- P wing
- Hammer Brother Suit
- Frog Suit
- Tanooki Suit
While Super Mario World only had-
- Mushroom
- Fire Flower
- Cape Feather
- Starman
- P Balloon(not as limited as the Kuribo's Shoe, but still a great deal)
- Yoshi
So yeah, 6 to 9. Also, the difficulty curve was a bit out of whack. Tubular in Special was smash-the-controller hard, while Way Cool was World 4 material.(even without the Yoshi Wings.) I never used the P Balloon in Tubular- I used a Blue Yoshi(cheating, but yes, even with that it was mind-boggling.) I also felt there was a lack of variety of level archetypes. There were only a few ice levels to my knowledge(Donut Plains Secret 2 and Awesome) and no desert, giant world, etc thing. Also, the colored Yoshis never really did anything truly noticable. Yes, you could use whatever power they were colored, but I think that could have been incorporated into finding secret gates better.
This is really hard to pick, I love them both so much and grew up with them. I might as well flip a coin to choose, but I'm gonna give my vote to SMB3 because the variety of power ups was fun. If Super Mario World at least made the cape not so uber and used a little more variety in terms of power ups and levels, then SMW would definitly win.