At your height you're gonna be right between a M and L
If you're going to be using the bike in big terrain (e.g. Scotland, Alps, etc) AND you want it to be flickable/manoevrable, I would personally go with the medium if you can get it to not be too cramped for the ups. …reason being that, once in descent mode, they are pretty long bikes.
When you tried the M Supershuttle and thought it was short, did you notice whether the handlebar was in its forward position, or how far back the saddle was on its rails?
I am 6'0.5" and have just got myself a Medium Bionicon Tesla. The Tesla is a touch longer than the Supershuttle though, cockpit-wise and size-for-size. It's comfortable but as small as I would want a bike to ever be for the climbs…but a lovely chuckable size for the downs.
Great bikes and great concept IMO.