I've seen a lot of random posts on this, so I thought about it and I figured I’d outline the actual differences I see.
1. Shifter difference. This is the most obvious and should probably decide which grouppo you get. I personally prefer campy’s thumb shifter system. It’s great for sprints because you can shift up quickly and separating the up and down shifting is smart in my opinion. I personally also hated the shape of the old shimano levers, but the new DA 10 levers I actually like, in fact I like how they feel more than campy. But the thumb shifters still have me hooked. In any case, those are the differences between the two and it’s purely a personal thing
2. Shimano DA 10 crank. I’ve heard from people that ride it that it is noticeably stiffer. Whether or not added stiffness actually makes a performance advantage is up for debate, but one thing is for sure: stiffer feels faster, and for most people, when you feel faster, you go faster.
3. Availability of parts. This is especially a problem in the US. Shimano parts are just plain easier to find when your bike breaks in a random town the night before a race. Not so much of a difference in Europe but its still there
4. Chain. The new DA 10 chain just plain sucks in my opinion. It’s a new special super-thin width, even thinner than the campy 10 chain. It's just screaming "break me! torque me!". Now I also think the campy 10 chain sucks, I torqued mine after only 2 months, but at least you can replace it with a wipperman chain that will last a LONG time. You can’t do that with the DA 10 chain because of the special width you need to accomodate the super-close DA 10 cogs.
5. Price. Less of a difference now with DA 10 being so expensive, but in general, campy is a bit more expensive than Shimano.
6. Coolness. Campy is cooler. Admit it. Lol well actually I think the new DA 10 crank looks cool but Campy still has an Italian artistic flair. Plus it has carbon, which automatically makes it 10 times cooler. lol. Shimano is so mechanical. But this isn’t a difference like the others in the list, just me flaming lol.
My personal opinion? I’m sticking with campy cuz I like the shifting and hate the DA 10 chain. But my opinion doesn’t, or shouldn't, matter to anyone else. Anyways, I figured I’d outline the actual differences for the hell of it.