The 12:1 box will be quicker reacting with fewer steering wheel turns from full left to full right, or lock to lock, probably around 2.5 turns or so. If I recall correctly, that 12:1 was part of the GM F-41 suspension package in the late 60's and early 70's. The 20:1 would likely be around 3.8-4.0 turns lock to lock. Standard type ratio.
http://www.chevelles.com/techref/ftecref29.html Read that article. Here is an explanation from it: With modern radial tires and performance suspension enhancements, many enthusiasts would like to improve their steering gear ratio to a rapid 12.7:1. The steering ratio of a steering gear is the number of degrees that you rotate the steering wheel (and therefore the gear input shaft) in order for the output shaft to rotate one degree. The lower the ratio number, the faster the steering. This paper is written to assist Chevrolet Chevelle, El Camino, and Monte Carlo enthusiasts in swapping their slower ratio power steering gears for a fast ratio 12.7:1 model 700 power steering gear. Please note, there is a new Saginaw model 600 power steering gear that is also available with a 12.7:1 ratio. However, this gear is not interchangeable with the model 700 gear.
