Not to mention that PHP is installed on 99% of web servers whereas ASP is less common (I know my webhost doesn't have ASP installed, and doesn't support the installation of it). I think it would be best to learn them both, because there are things that one can do that the other can't; but I would start with PHP.
I disagree with that comment. They both do te same thing but in different ways, you can make 1 do exactly the same as the other and vice versa. As for which is best i think PHP is the better purely because its more common.
Well, since I don't know ASP (hard to learn it when your server doesn't have it) but I do know PHP and I know that in ASP you can stop the header from sending requests when it refreshes (so that you can run a file upload and still refresh the page to display a status bar of how far the file is uploaded), and as far as I know, PHP doesn't have this. I suppose I should have added that they are almost identical in what you can do with them.