IMO coils are a far superior system than leaf springs for towing and general drive-ability. I've been in my 2500 Ram for nearly 5 years so my memory of my last truck is fading but I remember when I first purchased, the towing improvement was pretty drastic. The leaf springs were jarring and bouncy while the coil springs are just more smooth and consistent.
With only 1200# you will have very minimal sag. It you ever go heavier I highly recommend air bags as a supplement. I tow an 11K 5th wheel which puts about 2500# in my bed (250 for the hitch + the pin weight of the 5th + some gear in the bed). Even without the bags inflated the sag is minimal with the coils.