Based on your back/hip problems I would think you would be better off in the aisle seat in the lower deck. The stairs to the upper deck is pretty steep, and you will need to go up/down the stairs as you will not be boarding directly into the upper deck of a BA 747, unlike on QF A380. If I had a hip and back problem I wouldn't particularly like to go up the stairs, personally.

It's worth noting that BA J non-aisle seats require less stepping over than on QF J seats on A380 though. This is due to only needing to step over the footstool rather than sleeping people's thighs/knees as you would on QF.

I find that BA J seats are more flat than QF J A380 seats - QF A380 J seats seem to have quite a noticeable droop.

Window seats on BA J cabin tend to fill up more quickly as they are very private and great for solo travellers, but it's probably unlikely that you would be unable to be seated together.