I was doing GLA-DXB-PEK regularly the past 2 years. The DXB-PEK leg is a fairly quick flight but always managed to get a few hours sleep ( this was on the A380 which arrives in PEK in the afternoon). The 77W flight arrives later in the evening and the added benefit compared with CX is that you will get CD transfer.
I have arrived on the later CX & KA flights from HKG and it is a particularly busy time at the airport especially if trying to get a taxi. The last time I didn't bother staying in the queue as it would have probably been as long to get a taxi as it would have getting the train/bus to my accommodation. With EK, the CD driver will be waiting for you and take you to your hotel/accommodation with minimal fuss especially at that time of night and you should be ready for a decent sleep. For this reason alone, I would choose EK.
Same with the return, you will get CD and helps with the language barrier and trying to get transport to the airport. Despite the number of taxis in Beijing, it can be quite difficult at times to get one. The flight back to DXB is usually longer and my last two flights were as long as the advertised flight time just about so schedule padding for taxi etc was eaten into by the actual flying time which did maximise sleep time.
Some other pointers. Make sure you have a VPN pre-installed on smartphones/laptops/tablets. There are many websites/apps blocked by the Great Firewall of China and sometimes can be frustrating if you want to access certain email domains and can be problematic if trying to get a VPN whilst in the country. If you are going to be going about by taxi when you are there, download an app called Beijing Taxi. It translates your destination and most taxi drivers will be able to get you there without any problems.
If I can help you with anything else PM me or post here.
Safe & Happy Travels
S