I would go with RCI
I am a Blue Whale owner also. I have owned two 1BR units there for about 6 years or so and have used it exclusively for trading, though I think it would be the perfect place for a low-key, relaxing beach vacation.
We deposit with RCI and when we are able to reserve summer weeks at the Whale we get awesome trades. I mean, awesome. You are definitely behind the curve if you want So. Cal for this summer, but I still think RCI will be more likely to come through for you than Platinum Interchange or another small exchange company. Better yet, if you can find some other way to get a place for this summer, use your week to submit a search for NEXT summer (or Christmas, or whenever) to some highly desired destination. There are no guarantees, but I can assure you you have a great trader there.
As for bonus weeks, I respectfully challenge Lee's characterization that the owner he's referring to was "getting away with" something by getting bonus weeks from RCI. When I purchased my weeks, I was told there is a standing agreement between the Blue Whale and RCI that for each BW week an owner deposited with RCI, RCI would give the owner a bonus week. The bonus weeks are good for ~1 year, and cost $99 to use, and can be applied to a search (although you can't do this online, and the bonus week has almost no real trading power). I have gotten two each year, one for each week I deposited. I know of other BW owners who also get bonus weeks. If there was no such agreement between the resort and RCI, I am sure that RCI would not be handing out these bonus weeks. FWIW, they used to be a nice perk, but in the past few years have become almost impossible to use. For the most part you could only use them for very last minute stuff, say, less than a week out. Even that seems impossible now. I used the last one for a long weekend to Queretaro, Mexico, in October 2004. I had 4 or 5 bonus weeks in my account until recently, when suddenly they just disappeared. I never bothered to call RCI and find out what happened to them. They probably expired, unused.
Hope this has helped.
-Corinne