I use Linux (as home servers, no GUI at all), Windows (I used to be help desk for Windows, now I focus on Windows Servers) and Macs. I have all 3 at home. I've been using Linux since Debian slink (switched to Gentoo after woody). I've been using Windows since DOS 3, including Windows 3.1. I've been using Mac since System 7 (although I used Apple ]['s in high school.)
I know of no peripheral available for Windows that isn't also available for the Mac (there may be manufacturers of specific devices that don't support Macs but there is always someone that makes the same thing that does.) The laptops and Mac mini are as upgradeable as other laptops. I've upgraded drives and memory in iBooks, PowerBooks, Cubes (all these use laptop components). I've replaced the ethernet daughter board on a Cube. On desktops I've replaced video cards, hard drive cards (SCSI, SATA, IDE, all are available.) There are no Mac motherboards available so that would be the only upgrade unavailable to the general public that Windows machines have. I don't know anyone outside of hardcore computerists that bother with this.
I've not done it myself (my mini is PPC instead of Intel based) but the current Mac mini is CPU upgradeable. This was not as easy in the past because Motorola/IBM didn't have PPC chips in stores like intel, but it was somewhat possible.
http://www.macintouch.com/specialreports/minimonster/index.html
For USB, bluetooth and firewire devices Macs are usually easier add-ons/upgrades than PCs. Most drivers are already included and usually pretty much work. The exceptions here are printers and scanners. They can be harder to do on Macs than on Windows. I typically purchase these from companies that have good Mac support. My general rule of thumb is that all printer manufacturers suck. big time. I hate printing problems.
Both Windows and Mac tend to be better than Linux at ease of adding stuff on. But Linux has come a loooong way in the past couple of years.
As far as the base OS goes, I prefer Mac. I like the UNIX underpinnings and security, I prefer the GUI so much more it isn't even close. When I boot into Windows most of the apps that auto-start are to keep the OS running (anti-spyware, anti-virus, etc....) When I boot the mac most of the auto-start programs are things I will be using the whole time. When I use Windows I feel like I'm constantly fighting an OS that was designed by a drunk committee. OS X tends to get out of the way and I'm not arguing with the damn OS all the time.
Apple's OS releases usually run faster on the same hardware and actually add useful useable features. Vista will barely run on hardware that is being sold TODAY. I can't think of any usable new features.
Both Apple Finder and Windows Explorer suck.
Internet Explorer sucks more than Safari, but Firefox is better than both. I prefer Camino on the Mac (it's gecko based.)
Applescript Studio in XCode is way better than Visual Basic .Net Express Edition.
Macs are price competitive with similarly configured Windows machines from large manufacturers (Dell, HP) but not with an el-cheapo built with the cheapest parts available.