I wouldnt worry about stem options if you go threadless, there are a billion options out there, perhaps moreso than going with a quill stem (unless you buy adjustable)
I personally am using an adapter because IMO it gives you the best of both - I can adjust height as I wish, if my reach isn't comfortable, I can swap out with moderate ease, and the same with angle. That said, I dont have 100 stems to cover all of my options... I got an adapter that goes from 1" to 1.125 & have the 31.8 mm handlebar.
Personally I'd go threadless. You can have the stem cut so you can use spacers for some height adjustability: spacers can be placed below and/or above the stem, just don't overdo below-the-stem. I prefer the bar clamps on threadless stems as you don't have to finagle the bars through the stem and risk scratching them: while the odds of causing a sufficiently severe gouge to raise stress-fractures on an aluminum bar are minimal, it could happen. A shim to use 1-1/8" stems on a 1" steerer is no big deal and works well.
If you're really a stickler for stem adjustability, Look makes(? used to make?) a ridiculously adjustable threadless stem with a couple of pivot-joints, for extension and height. Not cheap, but it's an option.
Here's a vote for threaded. The height adjustability of a threaded setup is huge for me. Plus it just looks better IMHO. As for headset wrenches, I think with any half-decent headset, once adjusted correctly you won't need to touch it even on the longest of tours.
Waterford RST-22, Bob Jackson World Tour, Ritchey Breakaway Cross, Soma Saga, De Bernardi SL
I went for threaded (quill) on my BJ. That was one of the reasons why I bought the frame. Threaded stems look better, IMHO, and are much easier to dial in the height. Plus if you cut a threaded fork and later try to sell the bike, it might be too short for prospective buyers. There is not a stem sold today that is nicer looking or higher quality than a Nitto Deluxe or Pearl. The weight differences between a Nitto and most threadless stems are minimal.
Quality threaded stems are very limited availability now, limiting choices in length/angle, expensive and relatively heavy.
I agree with Tarwheel in that threaded stems have an appearance advantage on C/V style bikes. I use Thomson Elite stems when I can get them discounted. They are quite good-looking for a threadless stem, and tops in strength/longevity.
For threadless all you have to do is cut the steerer extra long and add spacers above stem, then cut down later once your information/confidence improves.
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.
Bob Jackson World Tour (touring) and a Miyata 100 (commuting)
Thanks everybody for your answers (and for the picture Tarwheel, this was related to a PM)... Seems to be a matter of personal preference rather than a clear cut choice, but your comments have certainly clarified things for me. BF, you're BEST!
I wouldn't make it a major criterium in deciding on a bike. If I liked a bike, it wouldn't matter much. However, all other things being equal, I'd go with threadless. There are a bizzillion choices (I know - I've bought half a bizzillion.)