Its important to take a look at the software and peripherals u use regularly before making a decision. A lot of them, especially PDA and smartphones still do not have full compatibility with macs and this can be a pain in the A** when you need to sync stuff. Also, a lot of 3rd party softwares especially the more specialized/obscure/obsolete ones might not have a OS:X version that is mac-tel compatible. In terms of performance, the OS coding is so different from windows that its apple and oranges really. The mac is inherently faster simply cos the OS is not as resource hungry as windows. This allows for more resources to be free for other software. Also, a lot of mac software is still done by apple, some of the best in fact. This ensures 100% compatibility and hence having less issues. Windows does still hold the bigger market share and hence will probably hold the convenience card for now.
I personally have been running a powerbook g4 1ghz for the last 3 years or so with no problems. With 2gb of ram in it, i can do complicated filters in PS on 1gb files smoothly and as fast the last generation P4 wintels. Of course, having a external dedicated 40gb firewire800 scratch disk helps.