Vai is definitely a superb guitarist, very original with a truely unique sound. Jibboom is a good example of Vai as a 'shredder' (couldn't resist getting that dirty little word in there!!). I reckon Alien Love Secrets is his best cd, For The Love Of God is a great track, but I think Bad Horsie is a lot better.
Technically Vai is probably ahead of Satriani but I reckon Satriani plays more listenable music and tries more new sounds (even had a techno album).