It depends.

Location: Waterfront is closer to the action around the Pike Market. Ren is more of a business district requiring more walking to tourist sites.

View: Waterfront will give you nice views of the water. Ren will give you cityscapes with peeks at the water or the mountains.

CL: I find both lounges to be quite nice. I can't say one beats the other. Service in both is very good. Neither has weekend service. I've had luck with breakfast comped on weekends at the Ren.

Rooms: Both are very nice. Some people complain about I-5 noise at the Ren but I've never noticed. Once I was in the room next to the CL and did find that room noisy. The Waterfront is right next to the Alaska Way viaduct but I found the building very well insulated against sound. The Waterfront rooms seemed to be ever so slightly smaller.

I tend to go with the Ren due to easy I-5 access to get to the offices I visit while there. For night time entertainment it is also closer to the stadiums, which I go to whenever possible. If you're going to be there on holiday, you might prefer the Waterfront. The views are much better.