Can anyone tell me if there is any reason some cars have one over the other? I have the Bridgestones but it seems all the test cars in the writeups have the Michelins. Are they both summer only? Thanks in advance.

Michelin designed the tires for the IS-F, and at Lexus' request gave the specs to Bridgestone. Both tires are built to Lexus/Michelin specs and both are great. They are indeed summer tires.

Both IS-F and M3 come with Bridgestone and Michelin. They are both summer performance tires. I won't be surprised if they only last 10 to 12k before you need a new set. Manufacturers do not like to give business to just one tire company. They alway split them up