I agree with swlovinist that Super Mario and Alex Kidd are fundamentally different types of platformers. That doesn't mean that you can't compare them to one another, of course, but I think any judgement is tentative. Because they're so different, sometimes I'll prefer one and sometimes the other. Just depends on what kind of game I want to play.
Most of the time, though, I definitely prefer Super Mario Bros. As others have said, it has much more fluid and fast-paced gameplay. In fact, it's so fluid and fast-paced that it almost descends to the level of instinct. That's great and it's what I'm always seeking out in a video game.
Alex Kidd's gameplay never gets to that level of intuitiveness because its gameplay is just much more complicated and precise. Sometimes that's the kind of gameplay I'm looking for, though, and then Miracle World is my preferred game.
As for Wonder Boy (which I agree is a lot better on the SMS than Adventure Island on the NES), that's definitely a more similar style of gameplay to SMB, but if anything it's a little too fast-paced. It often ends up feeling frantic and out of control. Sometimes I like that, too, though.
...word is bondage...