Of course you'd expect me to say the Mach1 since I have one, so . . . get the Mach1!
Now, some of my reasons:
First the only thing the Mach really needs to handle equal (or better) than the S197's is a Panhard bar and a set of handling shocks and struts. That alone will do it.
The Mach1 can be made to handle much better with what is available on the aftermarket scene. The shorter wheelbase the Mach has will make it as good or better than the S197's, given you choose the right aftermarket parts. Granted, the aftermarket has flooded us with goodies for the new Stangs but the shorter wheelbase of the Mach1, matched to the correct suspension set-up, will always get through the corners faster.
The Mach1 has the 4-valve heads and I see that as a positive for the potential of developing more HP/TRQ than the 3-valve if so desired. As for stock for stock from what I have seen the Mach1 has a slight edge there too.
The Mach1 does still have the 28 spline rear and the S197's has the 31 spline, but unless you plan on launching from high RPM's on drag radials or slicks the 28 spline will hold up just fine.
I have driven several of the new Mustangs since they came out and everytime I get in one I say it feels like a Caddy compared to mine. It has taller, skinnier tires, and over mine it has softer springs and struts and shocks so it should.
The S197 is easier to get through corners than mine (because a longer wheel base is easier to handle/drive) but with the simple suspension mods I have done to mine makes it way faster in any comparable corner I have driven both models in.
The backseat in the new Mustang is supposed to have more room ?? but my head hits the roof when I set back there and does not in my Mach so to me that is debatable.
From the side the new Mustang has a great look but from the front it has a sad look and the flat hood looks too bland to me. The tail is a bit bland too for my taste but there are aftermarket fronts, rears and hoods that I do like for the S197's to fix that.
As said, the Mach has an identity all it's own and can not be mistaken for a GT or Cobra and I think it has the best look of the 3. When I bought my Mach I could have gotten the Cobra or the GT, but even with the power that Cobra had over the Mach, the Mach just had that Muscle Car look that I couldn't ignore. Also when driving the Mach lookng out over that hood across the shaker does something to me . .

. Of course there are shaker's available for the new Mustangs too so that could be done to it for a cost.
I like the way the Mach with an aftermarket shifter (that is a must to get IMO) shifts over the newer ones too, and even the new ones I've driven with aftermarket shifters doesn't have as positive a feel as mine does. I have to blame that on the new shifter not being mounted directly to the transmission case as the the Mach1 is and also a contribution to that is the S197's shifter is bolted to the body of the car as well as a long bracket up to the trans. This means when you are putting torque on the engine it twists to the right but the shifter will bind since it is mounted to the twisting transmission and the stationary chassis.
I still like the new Mustangs though and would like to have one. I probably will too, when it comes time to replace my current daily driver, an F150 truck but I would not get rid of my Mach1 for a new platform Mustang.
