I am on the verge of switching my home PCs into Macs. In fact, I have my mom on an iMac already. I know she cannot break it in any way. (No handle jokes or I'll bring out my whip! ) I restarted it tonight for the first time in 41 days because she forgot her password and I had to use a boot disc to reset it! I have her using a Mac because I know that she is not a computer-literate person. I know that if she got some random pop-up from the web or some trojan installed by accidentially clicking 'yes' on those tracker/spyware genre programs, she'd have a PC hosed in 10 minutes flat. Now, I'm not saying she can't use a PC, it's just that I don't want to waste my time fixing and removing trojans, worrying about virii, or having OS exploits being unveiled on a weekly basis. In many homes, this scenario is commonly found, at least in my experience as a computer technician and repair man.
Anyways, back on topic, like others have stated, cross-platform comparing is pretty much impossible. I've been a x86 user all my life, but have recently turned over a leaf when I started using OSX. To me, that is the selling point for Apple. Back to hardware, solutions like the AMD Opteron/AMD64, Xeon, Itanium, P4, and the G5 are all great processors. Each have their advantages and disadvantages, but the same can be applied to anything, really. There is nothing in this world that is perfect, unless you're talking about Brook Burke.
In regards to the lifecycle of a Mac, the hardware release time is far slower than PCs. This can be looked upon as a good thing and a bad thing. Good because you don't need to worry about your hardware being outdated as quickly. (this is platform dependant of course) In this scenario, if you buy a Mac this year, chances are it'll still be highly regarded 2 years or more down the road. As even AMD has proven, the number of MHz/GHz you are running isn't important; it's the balls behind them that matter most. As most of us know, clock-for-clock, all of the processors we've talked about in this thread differ from each other in this regard. In the 'bad view' of having slowly-updated hardware available for your platform, you may feel like you want something newer, faster, but there isn't anything for you to upgrade. In my time, I've met a lot of fanatics that enjoy the PC hardware and specifications changing every 3-4 weeks.
The reason I am considering changing to the Mac platform? I love OSX, the G5 is just damn sexy, and having the power of a terminal window connected to a unix core is very nice. Did I mention OSX is hawt?
My $0.02.