David,
I take it that you'll be placing the jerry cans on the roof? If so, I vote for the metal ones so that you can lay them on the side, rather than sitting straight up. Any amount of reduced air drag can only be good for the mpg! The link you posted above is a plastic one and it'll leak, guaranteed. Furthermore, the plastic cans may turn soft (due to UV exposure) and your ratchet straps will get slack. The metal ones, like the NATO style, have a tight seal and won't leak due to the pressure changes or bumpy roads. I normally carry mine inside the cab. I have no experience with Blitz cans.
If your cans are laying on the roof rack sideways, a clean way to fill your 80 tank is to use a jiggle hose. This way, the cans stay on the roof and you stay clean. If you end up placing the NATO cans inside the cruiser, place them on the driver side of the rear cargo area (build a little shelf and secure them to the wall) and then use the jiggle hose method here as well.
Anyway, just some random stuff to think about.