i've only ridden threadless on my ss mountain bike. so i can't say that i've had much experience in the threadless track bike area. unless you're really worried about weight or have problems adjusting threaded, then i think it all lies on the asthetics. i personally prefer threaded, because i think it passes better to the smaller tubing of steel frames (i also prefer steel). i guess if the frame was an aluminum w/larger tubing then a threadless might look better.
i don't have any problems with headset adjustment (of course threaded takes a bit longer to dis- and reassemble, but who cares). and as far as bar change goes, i have a diffent stems for my drops and bullhorns, so its actually faster than changing bars on a threadless, only one bolt and the bar angle is already set...