It depends. If you're making it for yourself and you can freely choose the platform, asp.net is a lot more powerful coming with a great framework. However php is a lot more popular among small businesses and thus among most host service providers.
If you're planning development of a product, again it depends on what is your audience. Small businesses tend to prefer php, while big corporations most definitely choose asp.net or j2ee.
target the small auidenice than bigger once, use PHP and it would be better for you in learning too as it is easier to learn.
Couple other things to make a note of.
Windows hosting accounts/servers cost slightly more then Linux ones. Therefore, building in ASP wouldn't really work, unless they had some funky mono .NET aspx apache module loaded. Doubtfull.
So to save costs, using a Linux host, and building in PHP would be easier.
Your also likely to find more information if you get stuck wanting to know how to do something. The way I see things is that PHP is more looked at, simply because of the cost implications; there are none!
ASPX or .NET is more charged for, simply because of the way its hosted and what tools you use to build with.
My advice, PHP.. Its sexual
I wouldn't say that PHP is easier to learn as opposed to ASP.NET. What it's easier to, though, is catching up bad coding practices. So when starting to learn PHP, don't even try to look for online tutorials. Get yourself a good book and read it. Once you're proficient enough, check out those tutorials to see the common mistakes that are done. That may sound rather harsh, but most often it's true.