Obviously if your business or leisure travel is in Beijing, Hainan will be more attractive than Cathay from Manchester. But if travelling onward to Chinese destinations, then choice will depend on price and quality (and I suppose these destinations are also available through the Middle East as well these days).
Either way, the first two times I flew from Manchester to Qingdao, it was via Amsterdam and Guangzhou with KLM and two China Southerns. It was OK, but I had two lots of missing luggage and the service was adequate.
Now we're spoiled. Last year I didn't have to spend money in a hotel in Manchester to have to get the first KLM, Cathay was fantastic, Dragonair from HK perfectly good, and on the way back had a good stopover in HK. Looking forward to doing the same in four weeks time. Hainan may or may not be cheaper to other Chinese destinations (but certainly quicker to Beijing) but I suggest they will struggle to meet Cathay's quality.