Javier is right, make sure to check the rental policies of whichever building you're going to buy in, especially if you plan to rent short term as many buildings have minimum rental periods (like 6 mos. minimum or yearly minimum). I find that it is usually easier to find renters short term on the beach, but downtown is also desirable.
Since it's your second home, you should consider how you'll enjoy it most when you're there as well, for example do you like to spend every day at the beach? Then go with the Belle Island, but if you're a spa person, Ten Museum's La Prairie would be awesome too. Good luck!