For manual steering the 19:1 ratio is used,
for power steering and GT's(manual & power)
the 16:1 is used. If you were to turn the
steering wheel all the way from the left to the
right (lock to lock) there is one extra turn
out of the 19:1 box versus the 16:1 box.
The 19:1 box would be easier to steer with
manual steering, most noticable at low speeds.
The 16:1 box yields 4 turns of the steering
wheel lock-to-lock. (assuming regular pitman
and idler arms, not the Shelby quick steer kit)
If the Shelby arms are also on there, it's about
3 1/2 turns.
With the 19:1 box you're looking at 5 1/4 turns
I had P/S in the coupe. I removed it for the same reasons. My box is a 16:1 ratio. I used the manual steering adaptor from Mustangs Plus. When I'm moving on the road, the steering is great and gives a performance feel but when I am almost at a stand still - it will strengthen your arms.
In the conv, I have m/s with the 19:1 ratio box. Steering is a snap whether I'm moving or not. The only downfall to the 19:1 ratio is that it feels like I am oversteering to turn the car in a corner since I am used to the 16:1 in the coupe.