Both have advantages. I would probably pick the threadless though. Threaded was great for my daughter and our other Trans America companion though. They were not sure what they wanted in bar height and it was so easy to raise and lower the threaded one as they changed theier minds numerous times in the first fe weeks.

I don't like the looks of threadless with a stack of spacers, but it isn't an issue for me because I like my bars 4-5" below the saddle.