I have never used the P540X, so I can't really say. The 400 HPT is a good camera, I am just not happy with the cosmetic issue, and also that the lens comes off, with it being an outdoor camera. I would have to say it depends on your application. We use these cameras to cover a wide area, zoomed out to around 3.6mm. If you are wanting to cover a parking lot or large area, then these are ok if you can live with the color change, and are putting them were it is easy to access if you need to clean the lens out. The other camera that we use is the G4-480DNS This is a really good camera that we haven't had any issues with. It is smaller and allows for more positioning with its mounting bracket.
Cory