I have just done some laps with these 4 cars around Road America to help choose, and have some advice for all of these cars to share.
Porsche
Very good car. This one seems to be the best of all three in High-Speed corners and good on the brakes. You will have a lot of fun with this car, which has plenty of grip everywhere. Lap time is not difficult to get from this car. Beware though that this car isn't the greatest in the transition from braking to the apex of slow corners, feeling very sluggish here. Get used to this, and practice weird lines in the overtake zones because it may be your downfall in wheel-to-wheel battles.
McLaren
The McLaren is easy to drive and get up to speed in. Sitting in the middle of the car definitely helps with cornering both ways and feels great. The best car on the brakes (even if it locks them) and very strong at the turn in for all the slow turns, seems quite happy to take the slower ones a gear higher than the other cars and has the torque on the exit to deal with it very well. This car is weakest in the high-speed and medium-speed corners though, so you may have to add some wings to deal with that.
Mercedes
This car is a pleasure to drive. Seems to be the best all-rounder. Stronger low speed than the Porsche, stronger high speed than the McLaren but never quite the best in any of them. Feels like a car you can easily find time with by just pushing everywhere until it all hooks up. Doesn't have a Gear indicator on the dash though so you'll have to have the HUD open to see if you're not using an H-Shifter.
Nissan
Brakes are terrible. The big braking zones like Turns 1, 5 and 12 are actually OK, but it's the short braking zones like Turns 3 and 14 where you will notice this most. Deal with this and you can drive it happily through the faster sections, but you just know that there's a 3rd gear corner somewhere that is going to ruin your day.
Overall, I'd say the Porsche and Mercedes are fastest and very equal. The McLaren is just a little behind them and the Nissan should only be used if you're a Nissan fan, or a quick driver wanting to BOP themselves back into the pack. That said, most of these issues could be easily tweaked in the setup if you know what to do I reckon, and you can just find plenty of laptime anyway by doing laps. I didn't notice fuel usage or tyre wear, so I will leave that for you experiment with and work out.
Despite all of this, I was able to get all 4 cars within a second of each other just on default setup. All these cars are beautiful to drive and very well matched. If you want to make a decision without having time to do laps in all the cars, what I wrote above will help. If not, do these tests yourself and work out which suits your driving style the best. otherwise, choose the prettiest car and livery and just go for it. I can see that all of these cars could win the race in the right hands, so don't feel like picking the car I chastised is a death sentence to you being competitive in the race.