PHP is the technology for the masses; by some reasons, it is the most popular, is always available even with the cheapest and even dirtiest hosting plans, and so on. So, this is the cheapest thing. But it is probably the most of ad-hoc kind, most careless; PHP is not a serious programming language, looks more like ersatz to me. Well, you can leave with it, depending on how serious your problems are.—SA
ASP.NET is about the opposite pole. It's probably the most advanced and flexible, but it's harder to get, hosting is typically more expensive. Also, it is almost exclusively based on IIS, which doesn't have such good performance as Apache. (Apache-based CLR module based on Mono is also available, is based on the module "mod_mono", but it's so rarely used, hard to get, so I cannot give you enough detail and judgment.) With time, the situation is gradually shifted in favor of ASP.NET; there are more and more developers, you can get more of good advice on programming forums, and so on.
Somewhere in the middle, I would also consider some frameworks based on Python. I used to like it, but did not have any experience for last years. One thing I like was that Python could be used for both Web programming and Linux scripting. But here, in the valley between .NET-based frameworks (it's not just one) and PHP, there are too many alternatives, so it's hard to review them seriously in some Quick Answers.
Anyway, don't take any advice too seriously. You really need to make your decision based on your goals, experience, available resource and other factors.