Pros and Cons of QFN and BGA packages
Beside what is already said in comments,
BGA has the disadvantage, its hard to replace the IC by hand (but possible)
Advantage is, HF Signals like High speed busses can be guided directly from under the pin by special vias into the inner layers, which from impedance is much better than with QFN. Its a good Idea to do a raw routing concept for both and see if BGA is an advantage in your application.
In any case you can place critical components with less distance to an BGA than a QFN (the drawback is, you need more testpoints on your PCB to have chances for production tests or you need a good boundary scan concept)
In projects I participated the point to switch from QFN to BGA was around 80-100 pins...
BGA also normally has a PCB inside to map from your die to the balls, which costs some impedance and don't expect the manufacturer to get it right at the first shot. Advantage is, its possible to bond two die's with no real more effort.
I cant remember which is better, but heat dissipation also is a point to consider