I've recently converted my USS bike to OSS, not because I didn't like USS, I just fancied a change.
USS advantages -
slightly less twitchy low speed handling
end of handlebar is a convenient place to hang your cap on those tiresome climbs
MUCH, MUCH easier to mount computers, mirrors etc
Bike is much easier to store / fit through doorways
Also, with my hinged tiller setup I can raise or lower the bars a bit to stretch my arms.
I find both setups equally comfy but on the whole I think I prefer above seat steering.
Has to be USS for the majority of us - because lets be honest, we're not very fast are we? That last 2% of aero isnt making that much difference.
USS complements the whole laid back, falling asleep in the sun, getting a tan, confusing people who've never seen a bent before, "it might be an ergonomic miracle but mainly its fun" thing. And thumb shifters are cooler.
I converted my OSS to USS out of curiosity, & rode D.Tek's fleet, so my opinion is vaguely respectable.
My Hurricane has a long upright section between a hinge and the bars enabling the bars to fit above the legs and (as above) allows positional changes.
BArs can be narrow, simply enough to hold the controls, or norma handlebars
Others like the speed Machine have a shorter "stem" with large sweeping bars which surround the knees