I can't tell you about the ride, but I can attest to the loaded/unloaded problems. If I put enough leaf spring on my car(46 Ford coupe) to carry a load of people and car show stuff, it sits too high in back when unloaded.
Not to mention the stiff ride.
If I was doing this thing over, I would put air on it just for that reason.
I've been considering doing an air over leaf type setup on the rear to cure this problem.
Go with the 4-bar/air setup IMHO.
Oh, don't forget to add good sway bars(anti-roll bars) front and rear if you do add air.