Quill, if you desire the bike to retain it's classic appearance. How would you put a threadless stem on an older bike that wasn't made for it? You can use a quill adapter to hold the threadless stem, but the only advantages you are gaining are stem choice and ease of changing out bars/stems in the future. I have one of those adapters on my tandem as I couldn't find a quill stem in the size I needed, but I could get a threadless.

I have an '88 Schwinn Prologue. It has a quill stem with one of those more modern oval shaped stems with a removable plate. It just doesn't look right, though getting the bars off is easy. I recently traded someone for an 80mm old style stem to make it look right.