As the owner of both, my answer would be, what are you looking to get out of your car.
You're talking about the best balanced versions of what was offered in each platform. Neither were the fastest of their kind, as there were blowers strapped on to each generation and snake logos affixed.
My first thought was, when I got my Boss, that this was similar to the differences in a '69 Mustang and a '73 Mustang. The Boss drives 'bigger', is bigger, has more power. It feels like it belongs to the video game generation, while the Mach 1 is more communicative, feels smaller, more nimble, more strapped on than strapped in. I'm often asked which I prefer.
My answer is, I like my Boss, I love my Mach 1.
