You can't really comapre a pure midengine sportcars with a 4 seater ponycar. Totally different cars.
I own a C8 and it's honestly the most impressive car I've driven yet. The engine feels like so much more than the hp on paper, The cornering ability is just another level of what even modded pony cars can do. Material quality is leauges ahead of the Mustang (no material upgrade from S550 was my biggest letdown of Gen 7). I don't want to miss an HUD anymore. But while the frunk and trunk works measured in cubic-feet, just know that bigger objects just won't fit in or you have to puzzle a lot.
Mustang is a pony car, even if the drive is quite good in S650 with MR you still feel all the weigth transfer in corners and the engine just needs RPM to build up power. But you the trunk is really easy to live with.
Testdrive both and decide what you prefer. If you are looking for a car just for fun and weekend the C8 is unbeatable for the money. If you want a more pratical car with rumbling V8 the Mustang is a great choice.