If you're going to two shows, I'd say Vai and Stryper. QR will be around for a while longer. Vai will probably never play P&W in its entirety again. And, you'll never get to see Stryper again.
If it's down to one, I'd say Vai. He's the only one you haven't seen in one form or another. Plus, the guy is a monster! I've seen him at G3 twice. He's very entertaining to see live. And, that album is amazing! If I wasn't seeing Temple of the Dog at MSG within a week of the Vai show, I'd be there, too.