I use both Macs and PCs, and both Windows and Linux on those PCs. My subjective experience is that Macs are faster at some things, about the same on most others, and slower here and there. This is based on my experience with my 2.3 GHz G5 running Tiger, which is roughly equivalent to my 3.0 GHz PIV HT Windows/Linux system (running Windows XP and openSUSE 10.3 respectively).
With that in mind, Photoshop and iTunes are significantly faster on my Mac. Firefox and Thunderbird are about the same on all three of my OS' - it is fastest on Linux, where openSUSE has a very clever preloading mechanism, and slowest on the Mac. MS Office is about the same on my PC and my Mac.
However, the differences between Mac and PC are not very significant either way (except for openSUSE).
So, for equivalent speed processors, I would say it is more or less a wash. Faster in some things, slower in others, pretty much the same in most.