Dang it! I can't use either one! You see, I use MS-Paint to do all my detailed pixel work, and when you "Copy to," there is no option to save the files as PNG in paint. You can save the WHOLE file as a PNG, but not the individual 32 x 32 pixel squares that I'm working on. ...
It comes down to whatever makes you happy, but I agree with TechPro. Why use plain old Paint when you can do so much better?
And others user Paint.NET because they want the power of Photoshop (and not just the simple tools of plain old Paint), are using Windows, and Paint.NET is FREE!