Most hotels are in Copacabana and Ipanema. Copacabana hotels are close to the action (and the metro), but the neighborhood is noisier than Ipanema (which is itself noisier than São Conrado and Barra da Tijuca). If you plan to spend lots of time at the beach, stay at a hotel along Copacabana, Ipanema, or Barra da Tijuca (Copacabana has the advantage of being accessible by metro.) Note that "motels" aren't aimed at tourists. They attract couples looking for privacy and usually rent by the hour.
In the days just prior to and during Carnival, already peak-season rates can double, even triple. Expect to pay a premium for a room with a view. Many hotels include breakfast in the rate, but the quality varies from a full buffet to a hard roll with butter. If you're traveling during peak periods, make reservations as far in advance as possible.