I'm currently staying at the Grand Hyatt across the street from the Renaissance and down the street from the Marriott. We stopped by the Marriott today, and one of the agents let us view the lounge during the "quiet period." It's on the 23rd floor with an excellent city and Andes view.

We also checked out the pool, which was very small.

The Hop On/Hop Off bus begins a 5 minute walk from the Marriott - about a 10-minute walk from the Ren. All in all, this is a good location if you have a day or two in Santiago, since the Hop On/Off can take the whole day and is so convenient. The hotel will also book you a car/driver/guide to do day trips to the Central Coast, the wine country, etc.

The subway is about a 15-minute walk from the Marriott (assuming 15-20 minutes from the Ren), so if you plan to be in Santiago proper for more than a day or two and want the convenience of the subway right outside your door and more stuff in convenient walking distance, the RC is by far a more convenient location.