It's like VHS against Betamax.
The Mac is probably the more reliable system. Primarily because Apple keep a ruthless hold on manufacture and software approval. The trouble with that is that the choice is much more limited and the prices are higher. This leads on to the fact that many companies just won't write software for it as the market and the profit margin is to small.
The PC, on the other hand, is manufactured in thousands of configurations by hundreds of manufacturers. You can get a precise configuration to meet your needs. The down side is that there can be compatibility and integration problems. If you buy from a major manufacturer, however, there shouldn't be a problem.
Mirosoft has thrown billions into each new version of Windows. The rsult is that XP is a very good, very stable OS.
The Mac is still ahead in some niche markets, particularily in graphic design. So, if you're interests are heavily inclined towards graphics, video, DVD etc, I'd suggest a Mac. If, however, you're looking at games, office work or wanting to have access to a wider, cheaper, range of software - get a PC.