mustang_montana I think you meant 69-70

Those are great looking years, I prefer the 70' because the side scoops are gone and the headlights are more interesting to me. I also like most 70' year cars, its just my thing that started with my 70' Chevelle, my Friend Devin has a 69' Chevelle and for him its most 69' Cars!
As for the worth of a 73', its still held decently in value, and that car looks great, if there is no rust on that car and its original a collector will pay $20,000 for it in the right place. You could probably average a $15,000 dollar sale on it if you got it. Those year Machs are begining to show their worth as was said earlier. If it were a Cleveland 4V add another $2000 to the deal.
My wife owns a 71' Mach1 351C 4V C6 trans. The car is completly loaded except for the engine, not the largest engine for the year but by far one of the best performing. Her mother bought it new and left it to her after she passed away. Her car is worth about $30,000 restored, its got some work to go right now though. The engine is out being rebuild, complete floor pans (Her mom kept the windows down) and of course paint, no body work is needed but in one small spot. As it sits right now in need of total restoration it still brings a value of $4000 dollars. I think the only option that her mom didnt opt for was the spoiler, which I think made them look meaner.
Your Fox is worth pleanty and is for sure a hell of alot faster than this Mach1, but the Mach1 is an investment IF it is perfect. If it were me I would make the trade, find someone to sell it to and bring out more money then you expected from your Fox if you were to sell it. Though it will take some time to find the right buyer, it seems to take a while with classic, muscle and sports cars.
In the end its your choice, Good luck on that.
-Eric-