ASP is closer to PHP than it is to ASP.NET. If you're choice is between ASP and PHP, it's really just up to you, though if you're contemplating ASP.NET, I'd say there ONLY thing that should hold you back is the cost.
ASP.NET, IMO, is leaps and bounds beyond PHP. As John pointed out, ASP.NET easily trumps PHP for performance. Throw on top of that the cleanliness of your projects due to things like having a code behind model and user controls, well, it's just THAT much better.
If you're just toying around and looking to build quick simple sites, PHP is probably just fine. If you want to build larger scale sites and/or have more opportunities to work professionally, go with ASP.NET.