IMHO, it really depends on the Mach 1. I would not trade in an 80,000 mile car for one with 120,000 miles on general principle - the additional mileage means I'd be buying into worn out shocks & struts, bushings, bearings, water pump, alternator, AC compressor, power steering pump, brakes, rear end, etc., etc., etc. People almost never fix cars before selling them.
That said, your current car will need those items replaced in the future as well. If a significant number of those parts have been replaced on the Mach 1, the exterior and interior are clean and it drives tight and smooth and runs perfect, it's not that big a deal. You will spend more maintenance money over the next 40,000 miles on the 120,000 mile Mach 1 than on the 80,000 mile GT, but you're getting more car for your money as well. I cannot envision a scenario where you could get into a Mach 1 WITHOUT spending more money, IOW.
I would NOT trade a clean 80K mile GT for a rough, high-mileage Mach 1 just to have a Mach 1. I'd rather have a nice car than a beat-up rattle-trap regardless of the badge or day-dream potential.