What framework are you using for your PHP work? If you aren't using a framework (because your work doesn't need one), then switching languages probably isn't going to change much, but if you are, there are several good, fun PHP frameworks out there. The best PHP framework I've used is Symfony (http://www.symfony-project.org/), which is actively developed, fun to work with, and really powerful.
I've worked in CodeIgniter, but these days I find myself using the Zend Framework (http://framework.zend.com/). They're actively developing it, and it has so much built-in functionality that I've been able to get stuff done much faster than before.
My work just bought me a mac book pro, and coming from a souped up desktop running Linux, there were some upsides and downsides. Personally I would pick Apple hardware if you can get it and run OS X, but if you've never used Linux or OS X, odds are good that unless you really want to rethink how you use your computer, Windows is going to be the best choice for an operating system. One of the benefits of OS X, as others have mentioned, is the Unix core, but the applications for OS X are really awesome as well - Textmate, CSS Edit, Things. However, if you normally develop on Windows, the application centric vs window centric model of OS X takes some getting used to. Personally I prefer OS X's window manager over Windows, but that's just me.
