Finally got a chance to stay here and was able to clarify the lounge situation.


Main points:

1.
If your room includes lounge access, you can choose to have breakfast in the main restaurant for free ON ALL DAYS OF THE WEEK. The lounge does not serve breakfast on saturdays or sundays anyway, but I specifically asked the staff at the reception desk if this rule applied on all days of the week, and they said yes. This means if you are platinum (or above) you do not have to chose anymore between the 1000 welcome points or breakfast in the main restaurant. You get both.


2.
The lounge is open everyday, however, NO food is served at all ON SUNDAYS at any time.
On Saturdays, evening food and alcohol is served as normal.

Evening food time Monday to Saturday is between 6pm to 8pm.
Alcohol time Monday to Saturday from 5pm to 10pm


3.
Lounge is on the ground floor behind the breakfast area. Has about 10 to 15 tables. Not huge by any means. Evening food selection was limited, but acceptable.



Conclusion:

I found the quality of lounge food to be better at the Sheraton Zurich which is very close by. Also, the Sheraton's lounge serves warm evening food on all days, including Sundays.

If you are undecided between the 2 hotels and they are similiar in price, I would go for the Sheraton. The Sheraton's rooms are bigger, have very large bathrooms and have walk-in showers in all rooms. (My upgraded room at the Renaissance had a small bathroom and the just a bathtub.) Also you can open the window for fresh air at the Sheraton whereas at the Renaissance windows can NOT be opened at all. The tram stop is also closer to the front entrance at the Sheraton.

The Renaissance Zurich is not a bad hotel by any means, but in my opinion the Sheraton Zurich is just better in many different aspects.