[flycaliguy]

There is something that seems dumber about an "uh" in that it leaves the mouth open where an "um" seems more polite. By "um"ing and closing the mouth you are offering more of an invitation for the other speaker to chime in. An "uh" seems to fill space even after it's over.

[aric]

That's my first assumption. It's less strenuous and breathy to sustain an um, at least for me. There's a clear parallel to duh (like uh) usually concerning topics of [obviousness and stupidity] and hmm (like um) usually concerning topics of [deliberation and intelligence].