I would agree with what E.F. Grant has written about each. Ortberg is innocuous and not terribly profound, very mainline evangelical. That said, I enjoyed his book If You Want to Walk on Water, You've Got to Get Out of the Boat. It's not a theological treatise, but still a challenging book nonetheless.

I've heard Dallas Willard speak with his cohort Richard Foster about spiritual disciplines and agree with everything they're saying. I'm not sure he's Reformed; it seems he's a Baptist of some stripe, but I'm not sure.

Willard has been unjustly associated with the Emergent Church and I'm not sure why. Perhaps because he advocates the ancient disciplines of fasting, meditation on the Word, solitude, etc. The emergents love this stuff, but Willard is hardly part of the EC "conversation".

I got half way through his most popular work The Divine Conspiracy, and just ran out of gas. It's a good book but rather weighty. I hope to finish it some day.