I prefer the UD generally. On empty(ish) flights on BA however, sometimes it is nice to take the centre rear facing seats downstairs as one gets like a large(ish) cubicle to oneself if the next seat is empty. However those seats are the worst possible if there is soemone next to you...so, something of a gamble.

Also, when I fly into Lagos, which is not the most efficient or pleasant airport on the planet, I prefer to be on the 1st row on the left hand side of the aircraft downstairs. Every person in front of you takes circa 1 minute at passport control, and I have tended to find that people climb down those stairs quite slowly. Luckily for me the flight into Lagos is a day flight, so its no great handicap to sit downstairs.

But for night flights, UD is great for sleeping