I can not choose as both worlds are unique and different in their approach.
Mystara was great and the adventures we've had in Norworld are still vivid in my memory. It is also un Mystara that 6 characters became immortals and we've had a great fun playing litteral gods for a while. It was pretty challenging to build adventures for such characters but I managed to make them. It gave us a unique insight as to why gods act like they do and it still affects how I play them even today.
Dragonlance was a great world to have fun in. As a rule, just like any novel based game (Star Wars, Middle Earth, Darksword and so on) the goal is to either play way before or way after the events. This prevents the urge to replay what the heroes in the novel did.
Special note about the Kender. Most players got the Kenders wrong. Kenders are not thieves. Whenever a player wanted to play a kender in my game, I was systematically explaining that no attempt to pick pockets would ever succeed if it was for money. The kenders are as likely to take a nice looking teddy bear on a bed as they could take the diamond necklace beside it. In fact, they will probably prefer the teddy bear as if they give it the the Solamnic Knight, it might help that stuck up to sleep better as his sleep would improve so much with a nice teddy in his arms... what I did however was to give a kender to have a chance to have exactly the right item for a particular event. It was 15% + 1% per level. Ho we need acid, by the gods! I have just one vial here in my pack. I don't remember when I got it though. Or the caster would need a crucial component for a spell and by pure luck, the Kender just have that component in his pouch. A petal of a rose? I have a petal of lotus, would it do? (A petal of lotus in my games, at that time, would enhance the sleep spell by doubling the number of HD affected and upgrade the HD affected to 6+ HD, yes I was/am a Conan fan...).
With these little clarifications, Kenders were loved in my games and never hated as I have seen in other games. The Kenders are not thieves, well not in their minds.