I saw and loved them both.
As others have said, Dave feels a little wearier, a little more defeated. His passion for the piece comes across so clearly, and it's really incredible to see him put his whole heart and soul into it. His voice isn't perfectly trained like Groban's, but it's gruff and full of soul.
Groban feels a little more naive, a little more clumsy. Malloy and Chavkin have discussed the idea of Pierre as a sad clown- and that's exactly what Groban is. The world's got him down, but he still has that childlike innocence.
So if Groban is the draw, go for Groban. If money is an issue, go for Dave. You're in for a great treat either way :)