Depends on what you really want from your car once you start driving it. Sports car, or race car?
This conversation/debate/decision started with me in '97. Spoke to Liberty Racing Transmissions who sold both the T5Z, and Tremec who posed the same question to me.
The Tremec was a godsend to Mustang owners in the mid '80's who were breaking their T5's left and right. The factory T5's were only rated for 260 fp/tq to begin with. Stock 302's could do better than that. No wonder they were breaking.
The tall gearing on the Tremec 1st matched the T5's (~3.35), and Mustangs had 2.74 rear gears. Mustangs also weigh 3400 pounds compared to your Cobras' 2200 - 2400 pounds.
The close ratio (2.95 first) T5Z was never installed in any production car, so new is the only way you can get one. Liberty believed I would be happier with the T5Z for street use, even spirited street use, and an occasional 1/4 mile run.
I listened, installed the T5Z, and am exquisedly happy with it. Liberty blueprinted the trans for me, and installed the .80 5 th gear. Filled with Mobil
synthetic ATF, the trans shifts like a Honda, and is a joy to drive.
Liberty explained, the T5's weak link is the shifter forks. When these things bend during power shifting, the trans blows. Solution is use a positive stop shifter. Mine is a Hurst.
What you have working in your favor is a light car with street tires. The tires start spinning before you can put enough torque through the trans to damage it.
My current motor is stock displacement, but aftermarket everything else 351W. Like you, am seriously contemplating a 393 stroker upgrade. If it looks like I'm going to lunch the T5Z, I'll ship it off to G-Force for a gearset upgrade. I'll also upgrade the diff to a Powertrax or Eaton with 31 spline axles.
If you're going to be screeching the car at every traffic light, two things will likely happen. First, you'll spend far more money on lawyers and court costs defending your drivers license, than rebuilding your transmission. Second, you'll be replacing transmissions and other parts regardless of which trans you choose.
My recommendation for a mostly spirited street use car would be the T5Z with the 2.95 first, and .80 fifth. If you're bucks up, get the G-Force upgrade now. If not, there are other priorities that need dealing with.
Priority 1: Cooling system. The works here. Aluminum or brass oversize radiator, high flow water pump, high flow electric fan.
Priority 2: Suspension and brakes. Coil overs, tubular control arms, the biggest brakes that will fit in the wheel.
Priority 3: Wheels and tires. May we distract you from your 15" wheels and tires to consider 17'ers instead? Honestly! Drive the two side by side, and you won't believe the difference. It's not the same car! With 17" "Z" rated tires, you can use all the performance your 393 motor will give you.
Anyway, that's my suggestion. Food for thought.