I have flown 4x AC and 4x UA longhaul this year. If seat size and width is the sole factor, then I would say AC has a slight edge, the shoulder area is slightly wider and the footwells are slightly larger (comparing non-bulkhead). But with AC, you run the risk of irrops and delays, I've had 5 out of 6 flights delayed with them this year vs. 1 delayed flight on UA all year. I do prefer UA seat because of better privacy, better bedding and padding and I like my UA bulkhead seat