A dolly provides the smoothest shots, is very repeatable, easy to use and when combined with a short crane and sound knowledge of blocking and staging techniques will make your production value skyrocket. (Note: most homemade dollies are limited and not safe enough for use with an attached short crane.)
Steadicam is used when terrain doesn't permit a dolly. ie. Following someone up a flight of stairs. It takes a lot of time and skill to be good with a steadicam. I'm not talking about a demo shot either, but repeatable work for a director.
I use all of these tools, but they're meaningless unless you know how to properly block and stage scenes.