This is actually a loaded question. The answer is both. .NET is popular among a lot of programmers because it is strictly Microsoft based, and 80% of the large corporate population is Microsoft based. Thus, .NET programmers are in high demand to keep pace with Microsoft technology. This is also based on the fact that 5 years ago, ASP was the only real option for dynamic processing. Even if an open source version of .NET comes out, it will still be primarily Microsoft dependent.
Now you actually have three major players; .NET, PHP and Cold Fusion. While Cold Fusion is popular, it is not really a standard, and costs money. While ASP.NET costs money as well, most companies would much rather curb their investment and stay with Microsoft as that is where their foundation is based and the investment in changing an entire company would not be cost effective.
.PHP is extremely popular. One, because it is Open Source (free) and two, because it can be implemented on any server format (where .NET is limited to Microsoft servers). You will begin see the trend shift. PHP will become more popular with newer growing companies while the dinosaurs are dependent on Microsoft. You also see that 97% of all leased server space is UNIX based. This lends to the PHP/MySQL programming unit and increases the value of knowing PHP.
When it comes down to what you want to learn, while they are similar and you can easily move between each other without much difficulty, you have to ask what industry are you looking to work in? Large companies with a long history or newer companies with a certain amount of capital risk? If you enjoy the risk, then go with PHP as it is growing and as Microsoft continues to dig their hole, open source will continue to grow in strength.
I wish you the best of luck.