I'd book AUH. For personal reasons I prefer AUH over DXB as a holiday destination. Less bling without any loss on quality of product and a fraction of the price. From a BA flight viewpoint, you're guaranteed the Boeing 787-9 so you have the most up to date W/J product and IFE. You cannot guarantee this to/from Dubai especially if you opt for a 777 operated flight. Yes AUH is a sleeper service in J on the return so this will maximise the sleep time on that late departure. If you decide to upgrade using Avios you can also breakfast in the arrivals lounge at T5 as opposed to on board.