I have - and use - Windows 7 at work, Windows XP at work and home, Mac OS at work and home and Linux at home. I have also iPhone, iPad, Android tablets and more as a result of my job (I constantly need to test new platforms and devices.)

In general, they are - if you're just using web sites and doing basic stuff with digital images - identical. Pick the computer you like.

Here's where it gets different.

If you are the kind of person who likes to connect lots of external devices to your computer - game controllers, data loggers, and other fancy stuff, you're going to want to go with Windows based computers (and Windows XP if you really want to be sure it's going to have a driver).
If you're into computer games, no question about it, it's a Windows world.
If you're into selling on eBay, it's a windows world - the eBay applications that let you "mass auction" stuff are windows only.
If you like to be able to tinker with the innards of your computer hardware, it's a windows world (mostly).

When it comes to Mac, it's all about ease of use and excellent multimedia applications. There's no equivalent to Mac OS-only applications like Garageband, iMovie and Omnigraffle, to name a few. Every person I know who uses a computer to make a living - as in they spend their entire day in front of a computer and that's the main tool they use to earn their money - prefers to work on a Mac (even if work gives them a Windows computer).
Macs are more expensive out of the box, but I have found that they are also less fussy and get me up and running faster than a windows computer. I also find Mac computers to have some ineffable "unity" to how they work (same goes for iOs) - there's a lot of subtle and mature user experiences in Mac OS that only become apparent when you're using the Mac for a few weeks. Switching back to windows reminds me of these issues frequently, especially when it comes to file management.

Linux - in particular Ubuntu Linux - is an interesting beast. It appeals to two very distinct audiences. The first is the complete computer novice who will never try to do more than visit a web site and perhaps connect a digital camera to offload images. For these users, Linux is just a platform to run Firefox or Google Chrome, and that's about it. It's the ideal system to give someone who hates computers - you can basically set it up as a Kiosk running a web browser and it won't crash. On the other extreme is the perpetual tinkerer - if you want to totally customize your user experience, if you love twiddling setting and making a computer deeply personal, Linux is for you. You can mess with the innards of every program and with the operating system itself.

It's the middle ground folks - the fairly competent non-expert computer user who will be confounded by Linux.
You can't stream Netflix on Linux (no compatible media player). You CAN install SOME windows applications SOMETIMES with the help of a software pacakge called WINE, but they might not work quite right. Many won't work at all. Forget most Windows games.

External device support beyond printers and digital cameras is weak if you just want "plug and play" compatability. You can always write your own device driver if you can't make your device work. Digital camera support for lower end cameras is iffy. File system navigation is often confusing for people who don't really "get" files and folders. There are some confusing nomenclatures used if you're used to windows file/folder navigation.

But Linux is also free and had ALMOST all the software you could possibly want somewhere out there - if you want to install any software at all. With an Ubuntu distribution, you're almost set from first run. My Linux machines start from a basic installation of Ubuntu linux and then I add in Dropbox, install "Restricted Extras" (So I can play Flash video (Youtube, Hulu, etc.) and DVD's on it - you can't with a "stock" installation, and I use the Google Chrome Browser, which allows me to simply and without fussing synch ALL of my computers (and I have too many) with the same bookmarks, settings and all that.