I agree with JDot regarding West 8's proposal. It's a little gimicky, and beyond the arm-things the rest of the square seems like a cluttered afterthought.
I like Rosenberg's. The art installation has the potential to be beautiful and relevent, a true icon for the waterfront. Also, hers is the only design that seems to take into consideration the fact that Toronto's waterfront is pretty much inhospitable for at least 3 or 4 months of the year. I certainly wouldn't want to be laying in the sand, or playing with those arm things in the middle of January! Rosenberg's design can be enjoyed in the summer and appreciated from afar in the winter, with the added potential of winter skating for the reflecting pool.