I'd definitely suggest too that you try out both to see what suits you. I have no problem using either though my preference is still XP on a PC (then Linux) but I know some people who just can't use the one they aren't used to no matter how long they've been trying.
It also greatly depends on what you need it for. Macs are great for video editing but rubbish for gaming as most PC games come out for Microsoft OS's. That's just one example of many, what you need it for is one of the most important factors. Another important factor is that in the long run at least (don't know about the short run) PCs are cheaper as you can upgrade them bit by bit and you can buy your parts from anyone. When people upgrade a Mac they tend to buy a whole new computer but if money isn't a concern then that isn't a problem =) And then of course Macs do look better out the shop though for PCs you can make them pretty too with some effort. As for Linux, I like it but find little use for it outside of my studies.. it is good on a dual boot though. As for Windows versions you'd be hard pushed to get anything but XP these days and I wouldn't recommend any other. 98SE and 2000 are alright but XP is far superior (and it took me a long time to make the switch). ME is an IT persons nightmare :S
I started out on Macs but switched to PCs and I honestly will do anything I can to avoid using a Mac now even though I can use it fine. It's all personal preference (some people take it very personally indeed!) so sit down at a Mac and give it a whirl if you're thinking of getting one.
And wallpapers are the same for any kind of computer, they're just an image from anywhere you like that you can stick there
