Variables like:
Can a user have more than one VPN connection simultaneously per account (in other words, can you be logged on at home and also logged on via a mobile device simultaneously). Some will not allow this. SafeVPN does not.
Does user get dynamic or static IP address. some provide one IP address that you get all the time (SafeVPN is like this). Some provide a random IP address for each connection occurence.
Some sites are based in USA, some are not. Some have a lot of proxy servers, some do not. Some are very clear about where there proxy servers are, some are not. (Safe VPN does not seem to be so clear, beyond icons indicating USA and UK)
Some charge different rates for PPTP/L2TP, SSL or Open VPN. Some do not.
Some have software interface so that user does not need to do all setup, passwords, connections and options through Android Settings or Windows Control Panel. Some do not. Some have notification so you know the VPN is active. Some do not.
Some will support P2P, some will not.
Some have a monthly plan, some do not.
Some support Android. Some do not.