There is no right answer, I’m afraid! The seat on BA is better. If you’re someone who wants to maximise sleep, BA is better. On the other hand if you want to have a “premium service,” Etihard is far better. Unlike BA, you won’t have to choose between desert or cheese, for instance – have what you want! Salt and pepper are in pots – not paper sachets. Etihad try to provide a “luxury” service. BA provides a great seat but a product that is decidedly mainstream – meals, in particular.
Abu Dhabi is not the most comfortable airport to connect it – although I find it quite charming and like passing through.
I am a BA Gold Cardholder but, for me and the route you mention, would choose Etihad. But… I am not a great sleeper and I do like to feel “looked after.” If that was less of a priority, I would focus more on the seat… and BA would win.