I prefer the 25 and arriving earlier. You can get to your hotel, unpack, freshen up and then go out for the evening. Arriving on the 27 means your evening starts later and I find it more difficult to wind down - it is Hong Kong after all!

Having said that, my next HKG sector is on the 27, but that was mainly due to the BA Sale fare being cheaper in CW than the 25 at the time of booking. I'm also disappointed it is not a 747, as I love the UD LHR-HKG trip.