I just stayed at the Renaissance a few weeks ago. It was great! We got upgraded to Opera House view, and were on the 29th floor with Executive Lounge on the 30th. We had no problem checking in at 8AM, even though checkin time is around 2PM. The location is unbeatable. Its a one minute walk to the ferry area, with great views of the opera house and bridge (which are three-five min walks from that point). There is also a bus station there, as well as a train/subway/lightrail/thing station. The breakfast in the executive lounge was pretty good for being an executive lounge. And the breakfast down in the restraunt (buffett as well) was also great. I'd highly recommend it, although I don't know anything about the Marriott. Check out this board for more info on the two.