In what way? Apart from price, the machines are pretty similar.
Both have dual core processors, both have 4GB RAM, both have a 17" screen, both have a 320GB hard drive, both have a DVD-RW drive, both have built in webcams, both have a one year warranty.
The only area where the MacBook Pro will likely outperform the HP is in battery life, but then, I don't want to be one to fall for Apple's marketing garbage on that. I'm promised 5 hours on my MacBook, I get closer to 2 and a half.
For the reasons yetanotherdave listed, as well as the fact that you're getting a superior OS and an aluminum, unibody machine. As I said, the general public will most likely not be concerned with such things, but that doesn't mean they're irrelevant.