For the fun of it, I wrote a short comparison of Steve Vai vs. Nuno Bettencourt. Just my personal thoughts on this. I was going to write this at the end of the thread "underrated guitar player" (or something like that), that was about Nuno, but since that's a long thread and hasn't been active for a while, I decided to do a seperate thread about this:
The reason why I SOMETIMES agree that Nuno Bettencourt is up there with Steve Vai, is that it can be argued that Nuno is a better (at least cleaner) alternate picker than Steve and fast alternate picking runs that are also melodic are probably the hardest (or at least one of the hardest) types of fast guitar runs.
Nuno's tapping runs are also in some sense more melodic and rythmical than Steve's. I think that Steve improvises more with his tapping runs than Nuno while Nuno uses arpeggios that fit very well to the chord progression.
Both guys play with tons of feel.
Both guys play about equally fast.
Both guys can really make the guitar scream.
Steve is very likely the best (known) guy in the world with the wammy bar, wah wah effect and slides.
Steve put out "Passion and warefare", the greatest guitar instrumental record ever. Although Extreme's "Pornograffiti" has mind blowing guitar playing, nothing can live up to Steve's masterpiece, sorry.
Steve can be critisized for playing music that's too "weird" for most people. But that's just what other people like.
Nuno SHOWS more interesting rythim playing than Steve, although I'm sure Steve could play all that stuff as well.
Nuno can be critisized for his music being too "serious" (at least the last few Extreme records) and having an annoying vocalist (Gary Cherone) :) (Gary can sing well, it's just the way he chooses to USE his voice that can be annoying.)
Ultimately it comes down to the guitarist whose playing you LIKE more, i.e. the one who makes you feel better when you listen to him play. And either one doesn't make you feel "better" than they other, consider which feeling you generally like to feel more and you have your favorite guitar player.