I've reached the point in my noob build/fixie where I must acquire the 1 1/8'' threadless headset. I really can't afford or even require a high end; Hope, Chris King, etc, so have been looking at Ritchey & Cane Creek as well as others. My question is this: Most of the units I've looked at are spec'd for mtb use, but suggest they can also be used for road bike use. What distinguishes a 'road' headset from that for a mtb? Should I avoid those units spec'd as suitable for either bike? Thanks in advance for any advice your experience can afford me....

Headsets are headsets and as long as the diameter, type (threaded or threadless) and installation method (press-in cups or integrated) are the same and compatible with the frame.

FSA does make a "Pig DH" model that is particularly heavy and rugged as it is intended for downhill MTB use but it still works fine on a road bike if the somewhat clunky appearance isn't an issue.

I would recommend getting a cartridge bearing headset if at all possible as they are easier to install and maintain and tend to be more durable. I've had good luck with both FSA and Cane Creek headsets.

The MTB headset is usually built a bit heavier and is supposed to be more durable.

Just get whichever will work for your frame, since headsets basically never fail unless they're abused.
The least important bearings on a bike is the headset.

Not much, see what your head tube ID for what the insert is.. JIS is common .
Richey has a sealed bearing bottom race ball bearing top race , the bottom takes the hits.
so go for that .. cheap headsets that get replaced or posh ones that don't ..
either get you down the road..

JIS is not common and never has been except on earlier Japanese frames. All European, American and current Asian frames that use pressed-in headset cups take the ISO size for 1" threaded and threadless headsets.

For 1-1/8" headsets there is only one size so the JIS/ISO mis-match isn't an issue.

check out the velo-orange headsets. you can get a threadless cartridge cheap.

Good question, I was wondering the same thing myself. What I've found are MTB headsets are just heavier to take the beating.