This is a
very difficult question.
As you said, Ipanema is much nicer, trendy, than Copa. However, Copacabana Palace is a remarkable experience and you will never forget it. The Palace is consistently ranked as one of the world's best hotels, and has an atmosphere in its own. The Caesar, on the coutrary, is just a top hotel as any other top ranked hotel, but has nothing "special".
My suggestion: stay at the Palace and reserve the Caesar Park (or another hotel in Ipanema) for when you come back to Rio!...
Rgs,