I'm a realative noob and haven't paid much attention to this sort of debate, so perhaps I can offer a relatively un-jaded appraisal of my Ultegra shifters and a friend's Chorus which I just got a look at last week...
Right off the bat, the Campy definately feels better, I think becasue it's more slender (and I've even got large hands) but also becasue of the angle/curvature that it comes of the bars--I understand the DuraAce is more slender and better sculpted than the Ultegra, so perhaps Simano is making strides in the ergo direction. I also like the slightly "slicker" surface of the Campys which seems to make it a little easier to rock the bike while standing.
OTOH, the Campys requred you to come out of the drops whenever you wanted to upshift, while the Shimano shifters can be shifted in both direcetions from almost any hand position. This made the STI stand out, especially as my friend commented that he would sometimes get shoulder pain from the asymmetric hand position he would occasionally adopt in rolling terrain--a position with the left hand in the drop and the right hand on the hood.
Additionally, an unfounded objection to the STIs seems to be that you can accidently apply the brakes while downshifting--theoretically possible, but not inevitable, and no matter what, you're certainly not going to be able to apply any real "braking" force save for a brief rub of the pads.
I'd personally be happy w/ either grupo, but I'll probably stick w/ Shimano. Not only becasue Shimano parts are more readly available in the US (and often on sale), but also becasue Shimano seems more willing to innovate to meet the needs of riders.