Almost any up-to-date laptop is more than suitable enough for use by DJ's. The major requirement for a DJ laptop is an adequate amount of memory, with two gigabytes being an absolute minimum and a large enough hard drive to store a massive number of songs.
Generally speaking, an Apple computer is preferable to a PC simply because of stability and consistency. A Mac is typically less likely to crash while running any major software. However, when budget is a concern or when used for beginners, a PC can work just as well. Another thing to consider is physical durability, as a DJ's laptop tends to get more wear and tear than one for office work.