So my question is this, apart from aesthetic reasons (and a *slight* weight saving) what are the differences?
Short answer: "Not much difference in terms of use, possibly a lot in terms of aesthetics".
Longer answer: I faced a similar question when I got Sherpa and again later with my Nomad, as I wanted to keep both steerers long to allow maximum flexibility in handlebar placement and because I had designs on the space available (bottles mounted to steerer), and also wanted to know if it would be "safer" to have a low-mounted, upward-reaching stem versus simply flipping it for a horizontal placement atop a taller stack of spacers.
After doing a lot of math, analyzing the robust dimensions of the steerer (wall thickness and diameter), and a trip past Bike Friday's manufacturing facility here in town (they have successfully used much taller, smaller-diameter steerers on their Folders), I concluded it really makes no difference to safety or function at my weight and riding positon where/how a stem is clamped to a Thorn's threadless steerer.
As for aesthetics, a traditionalist might prefer a horizontal stem. Someone interested in saving every gram and certain of their position might wish to cut the steerer and go directly to the handlebars with a sharply angled stem. Placement of stem-mounted accessories will dictate the stem to be used (I found my GPS easier to read when it was mounted horizontally). Perhaps a person just likes one style more than another. All are valid reasons to choose one stem over another.
If the number or appearance of spacers makes for a visual hiccup, SJS Cycles/Thorn now offer an alternative: Tall spacers. My Nomad came equipped with these, making the whole of the steerer look much cleaner and more integrated. The spacers are available here:
• Individually (up to 48mm tall):
http://www.sjscycles.co.uk/thorn-alloy-headset-spacer-w-internal-bore-1-1-8-inch-prod27562/• As a set:
http://www.sjscycles.co.uk/thorn-alloy-headset-spacer-kit-1-1-8-inch-3mm-to-108mm-prod27590/They do wonders for cleaning up the "stack of poker chips" look that comes from using shorter spacers, and I really like the effect. Andy Blance very kindly took a couple photos as he worked with the Thorn crew to assemble my Nomad to allow the most flexibility in shifter placement with drop 'bars. I've attached these below so you can see the visual difference between the short and tall spacers.
Looking at the archived photos of your lovely blue Audax, I think it would look fine as-is or with either the new stem, tall spacers, or both.
Hope this helps.
Best,
Dan.