We've had this type of discussion before here, and it's always a hard decision.
To interject here for a second -- I think you might be asking "us" not so much from a practical opinion, but where your heart might be on this decision. If everyone said "oh, gawd, NO!!", you'd have your decision made up for you...(well, sorta..).
I can't predict where the '69 Machs will sit value-wise, versus the GT/CS. I think that the rarity issue will help make the decision., then again, I can understand the variety of owning more than one Mustang. I drove my GT/CS for 450K miles, and now I added a '94, which I call my "high-tech '68" (oh! no more window cranks!)
That "down the road" thing...marriage, kids, etc. You might think about which Mustang you'd want in your garage in 3-10 years from now. Income can change over time, and you might kick yourself for letting the CS go...and you'd want another one, and may not find what you'd like to get back.
I think it's not deciding on what's in front of you, but what you will want years from now. Can you borrow the Mach, for a long afternoon drive, and see what it does with your "spirit"? Does it exceed your gut feeling that you've had with the CS? (and think of the WOW factor you've had with the CS on the street...would you get that with the Mach? You decide.)
Maybe you should hang on to the CS, and save up for a late model CS?
It's all in what you feel, not so much what seem right at the time...or what's practical.
My two bits.
Paul N.