Our friends at Motor1 compared the "entry-level" McLaren 540C with the Porsche 911 Turbo S. Both cars fit into that nebulous area where we aren't sure if they are sports cars or supercars. Even the hosts noted that nobody will ever really know.
Regardless of designation, both cars are ridiculously fast. Both have well over 500 horsepower and are designed for you to whip around a track while laughing like a madman to yourself.
While it's not available in the United States, the McLaren 540C is an entry-level car in the same way that an F-16 is an entry-level fighter jet. Its twin-turbo V8 sends 533 tire-shredding horsepower to the rear wheels. Motor1 host, Alex Goy, threw it around the track with ease. Because that's what it's for.
The Porsche 911 Turbo S may be a different beast all together. It weighs a bit more than the McLaren as the other host, Vicky Parrott, noted while thrashing the blue ÜberBeetle around the track. The Porsche is also all-wheel drive, which gives it more of a hammer-like personality.
Whichever one you wind up picking, should you ever have to choose, it will probably be fine. Both cars are fine.