In SFO, the UA lounge is much better, IMHO. It's relatively new, has huge windows overlooking the tarmac, has snacks and if you are in C, you also get 2 complimentary drinks.
SQ is downstairs, out-of-the-way, somewhat dark and dingy.

Having said that, you can use either lounge if you are holding a C ticket, so give them both a try