Most of SB is fine, it's downtown Miami you don't want to be in at night. SB, Coconut Grove and the Gables are fine at night. Probably not going to find a good hotel on SB for that price. My advice would be rent a car, stay in the upper keys (Largo) and drive into Miami for cigars and a club. If you want nice beaches (quiet) and some snorkeling etc, stay on the keys. It's only about an hour from Largo to downtown.