Rack and pinion is a cut gear on the rack and a rotating pinion on the steering wheel shaft.
Rotary usually has a gear on the shaft turning a drum upon which the steering cable rides. The drum has grooves in the circumference to hold the spiral winding of the cable.
Both are positive engagement and somewhere around three turns lock to lock, but rack would be a tiny bit more precise. This doesn't make any real difference because of usual slop at the engine connection and some compliance in the cable itself. Dual cables make both more precise and able to handle higher horsepower engines.
So as said above, it comes down to room for installation. Rack and pinion simply takes more length under the dash while the rotary helm is a little bulkier vertically. You pays yer money, you takes yer choice.
Thanks everyone for there replies, I just went with what I took out and upgraded to a Dual NFB R&P. Installition was pretty easy, went in just like the old one came out.
from dual to single.... (again)???