I think you ride Marty until he loses. Give Miller a game in between, maybe the first Penguins game. Give Marty the second, and see how he fares. If he plays poorly, then go back to Miller. I can't see keeping a goalie out of the net that hasn't lost in three weeks regardless if he is a back-up, rookie, trade-bait, whatever. He hasn't lost.
Also another thing that bothers me about this situation that most fans don't look at. Marty is a real live human being. He has a lot of friends in that locker room and in the area. He knows his days are numbered. Think of that day in and day out starting in August til now. Its a heavy load that he has to carry on his shoulders. It won't be a surprise trade like that of Joe Thornton, this is a definite trade in the future, the question is WHEN. I feel for the guy, because athletes, are people too and i don't think any of us would work especially well knowing that we were only a day or two or three away from being transferred to another division inside the respective companies we work for.