It's quite true that Mac has superior industrial design and a much more stable OS (vis-a-vis Vista), but unless you get the uber high-end Mac Pro, your options of tinkering with the innards of the system is rather limited. And gawd forbid you should take a crack at overclocking...
In addition, there is also the "bang for the buck factor" to consider. I can get a lot more hardware for the same price with a PC - especially if you don't mind getting your hands dirty and actually do some research/hands on work with your computer.
I do think that for the majority of consumers, Mac is actually a better choice than PC because it demands less technical savvy on the part of the user (and that's not a bad thing at all - especially when say your mom calls and ask you about the BSoD)
re: XPS
I've just ordered a XPS 630 from Dell (waited till they are going to ship it with SP1 pre-installed - headaches that I am trying to avoid) - it's pretty reasonably priced for what it is and I'm thankful they went with standard ATX and left room for aftermarket upgrades. As nice as the exterior design of the 720/720H2C is, I can't stand the idea of having to go back to Dell for components, which they charge an arm and a leg for.
4K for Dell? That amount of money can get you some serious hardware (top of the line CPU, 2x SLI with top of the line GPUs, overclocked RAM, watercooling, 27" widescreen, etc). Yikes!
Now, when are we going to get
official OSX on PCs?
re: Desktop
Whether you go with a laptop or a desktop depends a lot on what you wanted out of your system. If all you need is basic computing (web, MS Office, etc) that doesn't require high performance and you don't need much, if any upgradability, laptop can be a very good choice - it's fairly cheap, with the added bonus of portability and not having to worry much about wiring, etc. Anything more and you might want a desktop instead for all the heavier needs (media, gaming, etc) and settle for a Eee PC for basic, portable functions.
re: Vista
It's not THAT bad from my experience - I am using it on my work laptop (Q2 2007 Centrino Duo w/ 2GB) and it's been fine for the usual, undemanding stuff. Fairly crash-free like XP. Speaking of SP1 - reviews suggests that it's a big yawn - though network file transfers speeds seems to have improved.
AoD