Michael Horowitz has recently published an article that does a decent job of comparing the ins and outs of VPNs. He highlights some of the fundamental differences between different types of VPNs and TOR.
Does using a VPN or TOR attract attention to you?
One of the more interesting parts of the article surrounds how VPN services and TOR exit nodes attract spies and police. Since those are the places where stuff people want to hide is being decrypted, it’s a great place to hang out. Using an IP address alone to identify a person is terribly inept, but that does not stop police from trying it. There are far better ways to identify Internet users without using just an IP address such as browser fingerprinting. Think of the potential of that deadly duo: collecting data from known points where people are deliberately trying to hide stuff, and then analyze it using fingerprinting techniques.