Most aircraft in the GA community dont have the performance to beat a mountain wave downdraft.
Depends of course on the serverity of the downdraft.
Best rate - most height in given time
Best angle - most height in given distance
It's a similar argument to blue or red line speed in a twin with an engine failure. My personal opinion is that if you have no other choice but to try and out climb it, and you are going down, use what ever attitude will give you the better RoC.
Pegging a speed that is not giving you the performance you are requiring is just plain silly in my opinion.