Glass is always better. However, the plastic carboys today (including the BMB) are pretty good at keeping out O2. In plastic these days, you really don't need to worry about O2 permeation.
One thing I always do when transferring to secondary, is add a few ozs of simple sugar. Boil in a few ozs of water, let cool (although not really necessary), and add to the beer in secondary. It will create CO2, and scrub any O2. It's not that different than when you add priming sugar for bottling.
We always worry about this and that when we do anything to our beer. This just takes one of those worries away.