The 215 isn't always a 747. Even when it is scheduled to be a 747, the aircraft type could change so it's not worth making aircraft type a major factor in the decision.

The OP is BAEC Blue and doesn't mention which cabin. 1A & 1K are nice in F on a 747 but the OP would not be able to reserve them. Some seats in CW on the UD are more pleasant but the OP wouldn't be able to reserve some of them, and would have to pay even more for the few UD seats that he could reserve than for seats on the LD.

I would plan your journey around the times that work best for you and not worry about the aircraft type.