Super Mario World had one huge advantage and one huge drawback. The advantage was you get Yoshi, a sweet dinosaur that jumped high and ate stuff with a frog like tongue. The disadvantage was that you couldn't stomp on goombas, and little things like that just didn't make it feel like a Mario game anymore.
Star Fox, on the other hand, was just all around awesome. The final boss was a little too difficult for my likings, but it's forgivable. It makes up for it with absolutely phenomenal dog fighting in space ships, the like of which would not be replicated for a long time. It was a very unique game that involved space travel, flying your craft through difficult terrain, and then engaging your enemies in combat.
Star Fox is the better game, I definitely vote Star Fox.