I stayed at the SF Marriott Union Square in January. I chose it specifically because of the location (proximal to a number of businesses that I was there to visit). I had a nice junior suite in the north-west corner of the building with great views of the Bay, Alcatraz, etc. Service was excellent overall. I'm not certain if the lounge is open on weekends, but it was well run & featured great views of southern SF. It's also right on the cable car line to Fisherman's Wharf, should that be of interest.
Don't think it is open on the weekends, but if it is --it is way better than the Marquis. Marquis' lounge is small, little seating, and food offerings are weak. I used to love the Marquis, before it becamse the Marquis. I don't care for it anymore. Service has gone downhill rather than uphill. Great views and suites at the Union Sq. Marriott also like the renovated Renaissance Stanford Court. The Marquis doe shave a great gym and a pool...FYI
I stayed recently and liked the Union Square, although the rooms are quite small, not unusual for the area. The concierge lounge was open on Sunday evening. Not sure about the rest of the weekend. I also like the Stanford Court Renaissance. A bit older but nice rooms and a very helpful staff. I had a delicious meal in the restaurant.