First off, Robert X. Cringely wrote a recent column on the comparative strengths of Linux and Windows. Here's a neat quote from the article attributed to Linus Torvalds:
"Because the software is free, there is no pressure to release it before it is really ready just to achieve some sales target. Every version of Linux is declared to be finished only when it is actually finished, which explains why it is so solid. The other reason why free software is better is because the personal reputation of the developer is attached to every release. If you are making something to give away to the world, something that represents to millions of users your philosophy of computing, you will always make it the very best product you can make. That's the reason why Linux is a success."