I have played and enjoyed them both. I took a hiatus from Minecraft shortly after he went "live"* with multiplayer to wait until he got some more bugs worked out and added in some more content, but I had a really good time playing it. Its fun to build various things from an 'on the ground' perspective.
Dwarf Fortress, by comparison, allows a much more involved take on your world, you can do so much that even having played for years now there are things I haven't really scratched the surface on (never had a King for example) and there is so much more in the way of stuff to do, things to find, ways to die.
As people have said already the two are radically different games, it would be like saying which is the better game Diablo II or SimCity, how do you even measure them against one another?
I will say that, unless things have greatly changed, I much more enjoy the DF community then that of MC, I don't know if its an age thing or if its cause MC has (over time) become more more well known in the mainstream then DF or what but people here tend to act much more mature and polite then over there, something that I sadly think is likely to carry over to multiplayer once I get around to trying it out.
I have spent far more time on Dwarf Fortress and enjoy it more, but that is primarily because there is simply so much more to do, in Minecraft you build your house, you play with switches or whatever a bit and build a mine cart, you kill some zombies and make some TNT and you explore the caves a bit and then you are basically done (again, I haven't played in many months so I'm sure much more is in now).
-MB
*Live in a 'sort of' sense as it barely worked and even when you could sign in you couldn't reliably even pick up stone, much less do anything involved.