Really good question. I don't have the hard data to offer a definitive answer. I would love for someone to point me to good, unbiased data, if it exists out there.
Of course, every situation is different, so even the best comparative data has to be filtered through the reality of any particular situation.
In any event, you would have to do some investigation, probably, to decide what's best for you. I would start with .NET if I were already a MS shop, didn't care about vendor & platform lock-in, and I'm not trying to create a massive web site (like the next Amazon, for example). I would start with J2EE if I'm already comfortable with Java technology and non-Windows platforms. (If I planned to deploy on Windows servers, I might lean towards .NET, instead.)
If I were going to create the next Amazon, or any other high-performance, high-availability application, I would spend the resources to evaluate what will work for me. It would be worth while doing pilot projects with both architectures. As part of those pilots, I would try to measure or "guess" the answers to your questions.
How's that for an evasive answer? ;^)