Not a lot in it, really - at least at legal speeds. Top-end, the 156 is quicker as it's much more aerodynamic (not hard, really....)
Not much in it from 0-60, but the 75 is much more flexible (2.0lt 8v twin-cam in the 75 vs 2.0lt 16v twin-cam in the 156 - 75 is all alloy, 156 has an iron block, 75 has 148bhp, 156 has, as I recall, 155bhp) - but the 75 is a good few hundred kilos lighter.
Handling is the thing that separates them - the 156 is awesome for a FWD car (most journalists rate them as one of the all-time best handling FWD cars), but you can never beat RWD with a transaxle (ie. rear-mounted) gearbox.
I'd not call the BMW 'cool', it's big, comfy and would survive the holocaust. The Ducati is slow, for a bike, with only around 72bhp from 750cc, but it's the sweetest handling bike I've ever ridden. Sounds absolutely amazing, too.