Three and the Brig together would have made infinitely more sense, and we would have been treated to the more abrasive aspects of their dynamic. I would have loved that. But I love Two, too --- he's such a sweet little guy, almost childlike at times --- so it worked well enough for me. I'm not sure why I like the Brig so much in this one. Possibly it's because he projects, better than the rest of the companions, "Perfect Companionism" --- he's capable and brave while still being all "What is that, Doctor?" just like a companion is supposed to be. It was just so good, he even surpassed the Doctors in my mind.