where to stay in vacation in mid December: copacabana or ipanema?

Copacabana ... I've been so I know it's for real.

I've never been to Ipanema and all I know is that some dishy young
woman came from there.

'If it's so good ... why did she want to leave?' I ask myself.

Hello, Livkle
I'm from Rio and I would suggest you stay in Ipanema.
It is prettier and more fashionable, with good restaurants and shops.
Copacabana is very near and you may go there whenever you want.
Keep in mind that Rio is very hot in December. If you're going to stay
for the New Year's Eve, then maybe Copacabana may be a better option
because the fireworks show at midnight is glorious on the beach.
Have a good vacation in Brazil!