I just went a few months ago with 3 friends and we actually had some trouble finding hotels which had rooms with two beds, believe it or not. A lot of hotels only offered rooms with one bed. So you might find that you don't have as many options as you thought.
We stayed at the National, which was pretty nice. It is in the northern part of South Beach, in the area with hotel beaches. A lot of the hotels on Ocean Drive looked fine from the outside but looked like they'd be shady on the inside. I liked that we stayed in a hotel that had its own beach rather than having to brave the crowds on the public beaches. And we were right near the Delano and Shore Club, which was nice for going out.
The Royal Palm also looked nice, and a friend who stayed at the Savoy last year recommended it but said that it was farther away than she would have liked from the places she wanted to go at night.
my friend stayed at the wave hotel last spring and raves about every aspect of it. and it's much less expensive than a lot of other hotels in south beach. (definately under $300/night)
I was there a few months ago and stayed at the Sagamore. I highly, highly recommend it. Haven't been to too many hotels in SoBe, but this one was great.
I also stayed at a suite at the Sanctuary which is on James St., a block from the beach, for a bachelorette party. I think this is owned by one of Madonna's close friends. It was pretty well-priced and the rooms were nice enough, with plasma tvs in every room. They have a pretty good restaurant and rooftop pool where they have parties and such at night.
HTH.
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Complimentary happy hr and they even shuttle you from the airport to the hotel.. Don't remember the price but I know we didn't pay more than $200/nite. Only thing is there was no tv in the room.. but we were out and abt so much we didn't need it..