In Economy I would definitely choose SK over UA on this route. Advantage SK: Widebody aircraft, better food and drink, often very nice service. Advantage UA: I believe there is VOD in Economy whereas SK only offers VOD in Eco Extra and Business.

On Star Silver: SK offers its own Silver members business class checkin and one extra piece of baggage. You probably would not get turned away at the business checkin with another airlines' Star Silver but I highly doubt you would be given extra baggage allowance. Perhaps others have more definitive info on this.