67-70 Mercury Cougar over 67-70 Mustang
Although I fully admit this is a judgement call.
First off, I find the Cougar styling cooler because the Mustang look is more common with Ford still riffing off of it. But I do agree the Mustang is iconic.
As for value, it’s pretty much a wash,with both cars being similar in pricing at the same option levels today.
But to me, the deciding factor is the engines. The Cougar was set a step above the Ford in pricing. You could option a Mustang to be as nice as any Cougar, but you couldn’t strip a Cougar to be as cheap as a Mustang. As a result, if you look at a Cougar of this era, it will have a V8, and most likely a 351 at a minimum. While Mustangs still sold a lot of I-6s back then. I think that all or almost all convertible Cougars came with Big Blocks.
So, for me, I would take either, but I would lean towards the Cougar as a starting point because I like the style a bit more and know that any random one I look at is configured for a V8, even if the engine is missing.
How often do you get a car with a front end that’s all mustache? That’s the Cougar. This car is your dad.