My small team had to choose between New Relic and DD and I found New Relic's billing model to be more appetizing. It was per seat and you could switch who was in the seat. Unlimited instances and most of the features were covered under that seat besides some extra things like HIPAA / Finance related stuff. They also have "regular" users that are free that can make dashboards and such. DD drove me nuts with their crazy amount of sales calls that just seemed to balloon.