Honestly, I don't think you can even say Petrucci in the same sentence as Vai.
Don't get me wrong, Petrucci is amazing, but honestly he sounds like pretty much every other shredder when he solos and I've been listening to Dream Theater for nearly 5 years now.
However, talking 'bout Joe Satriani, I remember once hearing this thing on a guitar radio show, and within about 5 notes I said "This has gotta be Satch!" and the radio announcer indeed confirmed my belief.
I could do the same for Vai as well, they have just so unique phrasing and nuance to their style, which unfortunately can't really be said for Petrucci.
As far as I'm concerned, Satch and Vai are the peak of instrumental guitar music. Sure, some guys can play faster, or whatever, but non one has ever been as expressive as these 2 guys and especially in Satch's case, usually never as tasty and restrained and caring for the music more than their egos.
Another thing a lot of shredders can't seem to do is play rhythm well. Yet Satch and Vai are monstrous rhythm guitarists too.
I do prefer to listen to Satch by far, because sometimes a lot of Vai's compositions are just too much and I really have to be in the mood for it, but both are amazing.
I saw Satch, Vai and Petrucci at the very first G3 tour in Australia, towards the end of 2006, such an amazing gig.