I'm now running both Vista and Mac and frankly, find each O/S to be "irritating" at times although in different ways . . .
This is the real truth, that is rearely acknowledged.
Both OSs are very good and both are tremendously annoying to use at times. And Macs are certainly not problem free. OSX has been a real PITA with a lots of bugs and issues. My Mac has completely locked up on me/crashed quite a few times this last week and has weird graphic redraw issues that I've not had since the 10.5.3 bug fix pack. And the mighty mouse regularly has problems with the scroll ball, other friends also have had problems with it too - there was a very good reason balls were removed from the bottom of mice as they tended to clag up.
As someone who uses both all the time, I do not see any need to move from one you currently use to the other unless there is some must have software that is platform specific.
I have however struggled on many occasions to find software as good as some that I regularly use on my PC. No matter what Apple fans may say, you have soooo much more choice with software [and hardware] with PCs. In theory you can use both OSs on a Mac, but I've not got Vista working nicely enough for my liking on the MacPro to justify adding any PC software to it yet.
My view is that as Apple control software and hardware, their computers should be much, much more reliable than PCs, sadly I have found the opposite to be true and it cost me a lot more to buy a Mac, for no real improvement and for file management and dual monitor handling a big step backwards in usaubility and functionality. Which results in a much inferior workflow. A shame as I like lots of other things in OSX
Also Vista 64 is a real 64 bit OS, whereas OSX isn't and CS4 is 64 bit on the PC, but not the Mac. So for maximum PS speed, Vista 64 is the way to go. LR is 64 bit on both platforms.