The Doubletree in downtown Miami is also a very good hotel, as is the Radisson. Stayed at both, and would heartily recommend them.
Tripadvisor is a very good place to start. SoBe (south beach) is the place to stay.
Try the hotels at Miami Beach [South Beach].
This is a lovely area of Miami with excellent shops and restaurants. We stayed at the Inter Continental in South Beach. I think it's on Collins Drive?.It overlooks the sea. Go Room Only tho as the breakfast prices are horrendous and there are loads of good cafes nearby.