Computer retail slave here.
A couple of different things to keep in mind when purchasing a computer.
First, the major differences between Windows and Macintosh. What are you mainly going to be using it for? School work, internet? Or are you planning to run the latest and greatest games? As a few already pointed out, the library of games for Mac is few and far in between. You can install and run Windows through Boot Camp (which is free), but you have to purchase a license for Windows on top of the already pricey notebook. If you're planning to do any sort of professional level graphic, movie, or audio work - a Mac is the way to go. They're still the industry standard, and although more and more companies like if you are proficient in both platforms, the majority of them want you to know Mac's first and foremost.
If you have the money, and gaming isn't important to you - buy a Mac. The machines are built great, last a long time (provided they're given proper care), are more secure and have generally less issues than a PC running Windows. If you have any issues, Apple will diagnose the computer for you free of charge...regardless of how old the machine is. If you purchase Apple Care on the machine, Apple also insures you for three years of coverage from any manufacturer's defects.
If you're looking at a PC, Toshiba and Asus have the lowest rates of failure, HP the highest, with the rest of the major brands falling somewhere in between the two.
Also, what else are you going to need to purchase with the computer? A Mac can get by without anti-virus software and iWork, which is essentially a single license Microsoft Office Home and Student for $50, $41 if you're a college student. On the other hand, you can probably find some really great bundle deals around the holidays for a PC.
Try to stay away from stores like Best Buy. The number of employees that actually know what they're talking about are few and far in between, and although they tote the idea of a "complete solution" over "selling the customer as many services plans as possible", they are still judged on whether or not they sell you $400 in additional BS that you probably don't need or want. Stay away from extended service plans through third party companies - if you want to purchase additional coverage, check with the manufacture.
Hope that helps~