Either way, shield or bag, it's going to be the lesser of two evils - you'll have to decide which one though.
Shield - better cooling of the turbo, though hotter underbonnet temps and therefore associated damage to the surrounding objects.
Bag - cooler underbonnet temps, the ability to 'touch' the bag after some hard driving, but heak soak into the bearings when shut down due to the trapped heat and no oil/coolant flow.