I'm no fan of Yngwie, but I saw him live in Dublin a few months ago and he is a serious player. I read a lot of posts saying that his technique is sloppy and that he is boring. Well, the technique was amazing at the gig. Very impressive, you could hear each indiviually picked note and the arpeggio sequences were flawless. However, by the end of the gig the solos did start sounding the same. There were a few high points and then a lot of the same thing, but he still put on a grea show.
I vote Petrucci as the better player though. He can play any style and always tries to play something "new". His technique is perfect and if you doubt his ability beyond Dream Theatre, check out Liquid Tension Experiment - excellent instrumental project with a lot of improv.