it depends on your requirement
An important thing to consider is the amount of video data you want to save and preserve.
80gb hard disk drive with 4 cameras recorded at 30fps(frames per second) is good enough for 2-3 days of recording
more cameras needed means a larger hard disk drive is needed.
also choosing the record mode. Will you use real-time 30 fps or lesser frame rates....
Software: most hardware based system will run on linux and will require firmware updates.
PC based systems will require a dedicated PC... usually windows base...