In some ways it comes down to how quickly do you want to have the 20%.
The "book" would say fund the 457 to the organization match, and put the rest in a fully funded Roth. That would give you some extra dollars toward the 20%. Then you could fully fund the 457 as well. This would maximize post tax growth.
I, however, fully fund the 457, to the detriment to the Roth. My reason for this is psychological, Out of sight (on automatic) out of mind.