Go with the D60. Youll be glad you did.
As far as a difference, the harshness of ride is pretty high with the leaf springs however I have nothing to compare to and I used Skyjacker 4" F-350 lift springs with mine which are famously harsh to begin with. Ive heard that coils are a softer ride.
But if you are concerned about ride harshness you should buy a cadillac.
If you go with leafs make sure you reverse the shackles. Then you can do away with the track bar and have a very clean set up.
Converting a D60 to coils can be done. Its easier on some years than others. Shilo can tell you what years. Like on my axle the diff is pushed so far over there is very little room for coils and radius arm. However, the beauty of Fabrication is that you can do "almost" anything!
My Superford still works!