It really depends. Both are worth learning. .Net is used in corporate projects. The salaries are a bit higher than those of php. Note that for the most part, you'll be tied to Microsoft, the biggest promoters of non-free, closed source software of questionable quality.
PHP, on the other hand, has a huge fan base. There are more open source CMS's, e-commerce templates and other intriguing applications implemented in PHP. Moreover, PHP is available in all systems (Linux, mac, Win) whereas .Net is not.
PHP is much better than dot.net. PHP is like JavaScript, VB Script and dot.net together, with an extra 700 functions. Definitely go with PHP.