Of the three, campagnolo is the one trading most heavily on name and looks. Mr campagnolo invented derailleur gears in 1642 or whenever, and the parts do look absolutely stunning... But then I hear tell that they don't always shift as cleanly and as said they're expensive to look after. Sram is the new boy, it's a bit exotic, it's generally the lighter of the three, it's fully user serviceable unlike shimano, where if the levers get borked you're boned. Shimano is everywhere, everyone knows it, it's a bit boring but it's highly effective. At the end of the day they will all pull a wire and let it go as necessary. At the price point for veloce/105/force they'll all be fine, it's just preference really.