Well, well. Since you have already decided the discussion remains only academic

and can fill pages after pages. But my last 2 cents
ASP.net like many M$ products will surely be very easy to use (although I have never used it). But recurring costs on hosting, etc. could be higher. Additionally, it would be pretty difficult to find free tools (like website maker or hosting) for it. If you value reliability, you will have to go for paid options with Windows hosting.
PHP on the on the other hand may be a little difficult (and more like C in coding). But consider the power of PHP and why people chose it. Let me give you some examples of web-applications made in PHP.
1. vBulletin (this forum is on vBulletin)
2. phpBB (Previous version of this forum)
3. WordPress (most popular blogging tool)
4. phpNuke (CMS)
5. Joomla/Mambo (CMS)
This list itself could extend to a few pages. So many people can't be wrong
And from the legality and morality point of view, php is free, asp.net isn't