Am at Pembroke now and can tell you that the second type of rooms you mentioned are either band c or b depending on which room you saw. The En suite ones are band D, which while being fairly expensive are nice, and having your own bathroom is an absolute god send! The rooms in the MAC (big ugly building in North Quad) are very "functional" but not really that nice, these are between band A and C. I would definately recomend a D band if you can afford it, if not go for a B as there is little to choose between B and C. However all the rooms are being re banded during the easter vac.
As for how they are decided, you apply for the rooms after you get your results and you normally get the band of your choice.