MUCH DIFFERENCE? Yes!!!!!
1) Veloce 2007 is down-sequential, like shimano (you can't shift from a 15 to a 12 with a single thumb-click, you have to do CLICK 15->14, CLICK, 14->13, CLICK 13->12). With Centaur 2007 if you keep a prolonged time youd thumb on the lever, you will obtain up to 3-gears shift down (also with chorus and record). Xenon and Mirage work like Veloce.
2) If you change your wheels, maybe the brakes are not perfectly aligned with the new wheels; you could move manually the brake (grabbing the whole brake system and assetting it on a new, centered position). Ok, that's not optimal if you screwed your brakes on a carbon frameset with a dinamometric key; so Campagnolo invented a system to regulate the brake axis with a very small screw on the side of the brake pad. You adjust it, without moving the brake, and you're ok.
In Veloce there are no regulation screws, in Centaur they exist.
3) Centaur is in carbon...much more fashion!!! Here in Italy I know many people that put on a Centaur 6-pieces and a FSA carbon crankset... then it's similar to a record groupset, with ergopowers, rear derailleur and crankset in carbon fiber