The phrase "which he prefers" is correct and usable in written English.
The phrase can usually be used when you are talking about someone's opinion or preference. For example, "Sam loves both Italian and Chinese food, but Italian is the cuisine which he prefers."
It's clear which he prefers.
He lives in Beijing & recently visited Shanghai, which he prefers as a city.
The tall, shaven-headed Scot sits on the floor, which he prefers, ruminating behind thick glasses.
He has a tendency to cut corners and a troubled past, which he prefers not to mention.
Aldridge, who faced more double teams last year, is able to play farther from the basket, which he prefers.
As Ruskin waxes forth about the shortcomings of Claude Lorraine, Turner's response is to ask him which he prefers: steak and ham or veal and ham pie.
He had become so busy as a doctor that he couldn't paint or think, both of which he preferred to medicine.
Another, which he preferred, was to be carried in a backpack and wrapped in ball bearings for, as he put it, "the casualty purpose".
Of the successive occupations washing over Latvia – the Russians in 1940, the Germans in 1941, the Russians again in 1944 – there is no doubt which he preferred.
She sees that town in her only son, who grew up in western Pennsylvania when the region boomed with its two most famous exports, steel and athletes, and he knew from the beginning which he preferred.
In 1936 Blum famously distinguished between the prudent exercise of power and revolutionary conquest of power — no one doubted which he preferred — while Mitterrand announced straight-faced in 1979: "Without a strategy of rupture the Socialist Party would lose its identity.
