Best of luck with the 911,
for me and most, it is slow, plain difficult, does not accelerate out of corners as quick as you expect of a rear engine hanging over the wheels car, and does not give the grip either under acceleration that you expect.
It is my favourite car driven out of liking it, not to be competitive.,
I would go with the McLaren, most drive it for good reason, I find it does not suit me, but on odd occasions I can be very competitive in it. Not that I am much cop , but it suits some circuits with me on occasions.
You will need to pay attention to getting a setup that tames it with respect to you?
Overall it is the fastest car in GT3, if you can extract the cornering speed and use the fine turn in to your advantage, which takes a bit of effort, it will reward you handsomely.
The 911 will frustrate you, a car with no advantages, apart from fuel consumption.