IMHO the essential elements of BA's First Class product (the quality of which has been discussed ad nauseam here) are the same on all types of aircraft, so if there is a good reason aside from the choice of aeroplane for you to pick one flight over the other, I would say go with it.

Regular BA First travellers tend to prefer one 'plane over another. 747 lovers cite the special feel of being in the nose cone, while detractors say it has the most cramped cabin; 380 lovers appreciate the slightly more spacious feel of the seats (I think the seats are the same size but the surrounding storage etc. areas are a little roomier) but (this is special pleading from a fatso passenger) the loo door is the narrowest!; the 789 feels special because it is such a small First Class cabin but there's only one loo; the 777 probably has fewest 'special' good points, some of the fleet is a little aged, and the cabin feels a bit dormitory like because it is large with no great variation over the rows. However the fundamental product is the same and often the biggest factor determining whether a First Class flight is great or not is the quality of the crew, which varies and is not dependent on the choice of aircraft. So I would say if the 777 timing is better for you than the 380 then go for it - the differences are too small to worry about too much.

P.S. tsmnc makes a good point about IFE - I always tend to forget that it is more aged on some of the 777s because I personally don't care about it, but if it's a big factor for you then it might be a reason for picking the 380.