I don't think you can really say which is better, they are just two different FS systems. One thing though, Specialized patented the four bar system, so other companies have to pay big bucks to use it, so you may pay more money for a four bar system than single pivot, but not because it is a better system.
What you have to pay attention to is how much the rear suspension gives when you are pedaling. Always go for the one that gives the least, or has a system where you can turn the rear into rigid.