I'd kinda forgotten, but the fact the Speedmachine *was* easily retrofittable was another minor advantage, because the frame's common to all models (ie the USS bar pivot is in the frame). Challenge, in particular don't (or didn't) have a common frame build, so whilst you could probably convert a USS machine to OSS (removing bars/adding tiller), you couldn't go from OSS to USS. Dunno 'bout other manufacturers...
(Pretty sure the Grasshopper can be swapped between types, probably the Streetmachine too - Kim ?)
I agree with Arvid re OSS being unintuitive - or at least, tiller is - feels weird having to swing a tiller and led to a minor off at low mph when I tried one
- but you do get used to them (not that I am at the mo'..). OSS does give you a *far* better turning circle.
Re arm position - OSS is more aero, but I've had some trouble in the past with ulnar nerve compression in one elbow, so USS suits me a treat. Yeah, it is a bit of a pain wheeling the thing around, having to stoop to steer, but I might not need to do so with the Streetmachine or Grasshopper, which are a bit higher.
I wouldn't get *too* hung up on 'normal' sized wheels - lower means its easier to get your foot down, and whilst they're wobblier to begin with (short vs longer pedulum effectively I guess), if you do come off it's not so far to fall (and less likely to bust a collarbone). Even the mid/lowish Speedmachine puts me at about the same height as the driver of something like an mx5.
Length *might* be an issue - I can't get my speedmachine (26" rear) into the OH's A4 Avant (hatchaback/estate-like thing) without removing mudguard/rack* or boom, whereas a 20" wheeled challenge Hurricane will fit (*some rack bolts are the swingarm pivot bolts too, so I'm not in a hurry to whip 'em out without a torque wrench and threadlock to hand...)
London Recumbents - very helpful indeed, sited in Dulwich Park so you can ride around without traffic
I went along to try a Hurricane, so they got me on various tillered things with various levels of recline/tiller offset so that the Hurri wasn't too much of a shock
They do hire, but I'm pretty sure since I was looking to buy I didn't pay a hire fee. Might be a bit limited to what trial machines they have, tho'
Kevin at Dtek is also very helful - he'll do a half day hire/fee session IIRC and you..ahem, your friend..would get to try all sorts of configurations, LWB, SWB, OSS, USS..
It really is a case of trying several things without too much preconception (other than any health issues, since that's how some of us come to get one), and see what suits - even my USS machine felt weird to one of the bods at Bikefix, since she was so used to tiller steering.