I have both a corto and a sadhana, though my corto is a "basic" so it's just the "slab" body without the additional contours in the wood.
They of course feel similar because the necks are the same spec (curiously, my necks are a little different in style - probably from different "runs", but still the same size)... The sadhana body is not as wide (as in, when you're looking at the front of the instrument) so it's somewhat smaller. Also, the sadhana upper horn attaches to the neck sooner than the corto... Doesn't make much difference unless you're the type that likes to wrap your whole hand around the neck to play chords really high up... You can do that a couple of frets more with the cortobass design.
Other than that, the differences are just what you see, different shape ("swoopier" look to the sadhana). They both balance extremely well standing or sitting (as you know). Most other basses want to slip, slide,neck dive, and otherwise launch themselves from my lap unless I hold them firmly by a strap and my fretting hand. The Birdsongs pretty much stay put on my lap just resting lightly against my body...
