I have enough CSC left this month to get one of the challengers. Which would be better to get for a tier 8 pet? The Discarded Droid Face Plate or the newer Living Petri Dish?

I don't think that a Tier 8 pet would be able to beat the Droid right away, but it would probably be the better investment for future battling. And it retires on the 1st, so you might be able to get more CSC for the Petri Dish next month while that one is still available.

The droid face plate is definitely the more worth while investment but you will have to wait until you are a higher tier before you can face him. The experience drop is definitely better from the C.A.P.S challenger and the challenger loot from the petri dish will definitely go down in price if you are patient.