I heard it described that Shimano products are always leading in the innovative area, and Campy is flawlessly crafted. Both are outstanding, if you get their better group sets. One other benefit to Campy is the carbon levers - they don't get cold like the metal ones. That might be useful on a cold race day.Personally, I use Shimano just because I always have.
Wow talk about opening a can of worms. You will find most people are die hards being either a Shimano or Campy person. Once they have been inducted into the cult of Shimano or Campy they normally will not leave.Now for the differences between the two, I would not say one is better than the other one. Shimano out of the box works flawlessly, smooth clean shifts everytime. The shifts are on the brake levers with the full lever acting as one shift and a smaller leaver behind it acting as another.Campy is a little rough out of the box. It usually takes about 500 miles to break in, but once you do its smooth as butter. The shifters are the brake lever and a thumb shifter on the hood.The big difference and one reason you see a lot more Shimano out there than Campy is because of price. A full DA group is $200-300(or was has been awhile since I priced parts) less than a Campy record base group. Now add in the Campy carbon record components and you can jump even higher.If your not sure what you would like go ride some high end bikes with both groups on them so you can get a feel and see what you like best.
You have some good answers here...Both camps have fans, and both are excellent. I have Campy and love it. I've had Shimano and had perfectly good luck there too.I would add that it does seem easier to get Shimano parts, service, repair, etc. More shops seem in tune with Shimano, but that could just be my area...