I would say that the kits for the original axle are about the same. The kits they sell for the six lug, in my opinion are worth the money. They generally give you a 12 inch rotor. The kits for the 5 lug disc brakes, generally give you an 11 inch rotor. They are roughly the same amount of work to install.
You may have clearence issues with the 15 inch rims and 12 inch brakes. You will have compatibility issues with the 5 lug brakes, and your 6 lug wheels.
Your stock spindles have to be in pretty good shape for both conversions. The 6 lug conversion uses your stock hub. You will need to seperate it from the drum to reuse. Now would be a good time to convert to roller bearings. The 5 lug kit has the hub integral to the rotor and comes with roller bearings.
If your king pins are worn or sloppy, now would be a good time to change those as well. make sure your entire steering geometry is in good repair as disc brakes will put more strain up front.
Last thing to remember... You can't just slap disc brakes on without changing the master cylinder. You should deside if you want power brakes or manual... both master cylinders are available for the advance design trucks. The new brakes will have to be "replumbed". New lines will have to be cut, bent, and flared to fit. Don't forget about residual pressure valves.
Good Luck!
