Jobst Brandt, author of The Bicycle Wheel and a mechanical engineer, says a threadless set up is stiffer. I'll take his word for it, but think the mechanical advantage must be minute.

I like threaded for the aesthetics, personally. As mentioned the lack of an open face plate is a big drawback if you like to change your bars a lot. But I have a feeling most people don't often switch bars.

Not to derail the thread, but doesn't anyone know of any good open face threaded stems? The only ones I can think of are the crappily made riser stems on entry-level comfort bikes. I've never seen one that had welds that didn't look like they were done by a third grader, a la the Dimension stem INK posted above.

I ran across these sweet threadless stems yesterday--just click on "stems" http://jonnycycles.com/