The main difference I've noticed is how they feed and why.... If you go to freewebs.com and look up sang vampire it'll bring up psi also and it shows some of the differences....
The difference is as aforementioned AND HOW they feed.. and HOW they get their energy.
There are MANY different types of 'vampires'
A sang is a blood drinker, and a Psy is a energy eater...
There is much debate over whether a sang vampire is actually gaining anything truly from the blood or whether the blood simply forms the psychic bond and in fact makes them psy vamps as well. I believe the Strigoi Vii (Sanguinarium.net) has now stated officially that due to the issues of blood borne pathogens and the safety of blood letting / blood drinking, they have ceased all promotion of blood drinking and are now completely of a psy nature.
And lets be honest, those who claim to be somehow stealing and drinking blood from a blood bank, could for all intents and likelihood be lying.
Well again that gets back to belief or faith in what someone is and whether the belief holds to a sang that uses blood for a connection or those that actually feel they gain something physically from the blood and not the person.
I have to say that much of what I see is lifestyle related. I know there are those who have a stronger spiritual belief to their identity and I wont argue with that but so much of 'feeding' is of a sexual nature to the majority of living vampires that I find it to be closely related to a mental fetish or blood play more than an actual sustenance of any kind taking place.
A psy. needs energy and a sang. needs blood in order to survive and maintain their health.