Regarding BA's allowance... It's brilliant but can be a nightmare for short haul, which is a what a lot of passengers do to connect to their long haul flight out of LHR. Great if you have a little status and can board early. BA's long haul jets seem to have enough room for most, but again it pays to be able to board early, just so you know your bags are directly above you. For me, BA lost its shine when it increased the passenger density of its short haul aircraft. Emirates? Can be easily gamed as long as you're not flying out of MAN...

With Emirates economy, I see the opposite problem. I struggle to pack all the things I want, that really should be in the hand baggage, within the Emirates limitations, but space is usually not an issue. If my next trip must be economy, then I naturally try to avoid Emirates. Of course there are always risks. BA has its higher density short haul cabins, while AF/KLM have have a decent official allowance while using FlyBe as a partner for short haul!