What is better, to develop my Android app on a Windows PC or a mac?
Subjective question. It all depends on personal preference. Do you like Windows and Linux - use WinPC. Like MacOS - use the Mac. Applications for the Android development in the language of Java, so the development tools for different operating systems exist in sufficient quantity.
Due to Apple's (to put it politely) "controlling" approach to development, you'll need a Mac to develop apps for the iPhone. So, if you think it's likely you'll be porting any of your Android apps accross to the iPhone in the near future, you might as well save yourself buying two lots of dev hardware and start with a Mac.
Also, emulating a Mac OS on a PC is hard work and against Apples ToS, but you can run a Windows OS natively on a Mac.
Because the API docs are not always complete, having access to the Android source code can be of great benefit. Unfortunately, setting this up for Windows is not currently supported by Android, but is for Linux or Mac OS. You can do it for Windows, but there is no procedure explained to do so, and it's not intuitive.
So in the case of easily being able to download and view the source, either Linux or Mac would be a better option than Windows.
I've developed Android on OS X, Ubuntu Linux and Windows XP, and in my experience it makes very little difference. It's also easy to chop-and-change. Pick the OS you prefer.
If you'll be using the eclipse IDE it will make little if any difference what OS you choose. Eclipse can run on a MAC Windows or Linux.