This is actually a tough comparison.
I agree with the point about Duncan being better than Garnett, and the two of them being better than all the rest.
My problem is who to place next. Ginobilli is the best defender of them all, and the best ball-handler. Allen is the best pure shooter, and definitely the one I'd want at the free throw line. Pierce is the best athlete. I could make a legit argument for taking any of them third.
And we don't even need to talk about Tony Parker, because - and I've said it many, many times before - he's incredibly overrated. He doesn't even belong in this discussion.
I'm going to go with the Celtics "big 3" as the better, simply because, in my eyes, the Celtics 3 beats the Spurs 2.