The 6-Offroad box is a completely potted unit therefore its vibration and water proof. This comes in handy if you're running the Baja 1000 or doing water crossings where the snorkel is above your head!
FYI - The Offroad box is obsolete and no longer available but you may find some old stock on a shelf somewhere if you find one.
The 6AL really is a great option as I've run one with no issues. I currently run a 6AL-2 prototype with no issues.
The 6AL however is obsolete now as well and will no longer be available so if you are buying a new unit I recommend the new Digital 6AL part number 6425. It's the best bang for your buck and it has a built in rev-limiter.
This unit has rotary dial switches in it to set the rev-limiter so the plug in modules are no longer needed.
It also comes with rubber mounts to absorb vibration so unless you'll be drowning the rig in water you'll be good to go. If you have any questions feel free...
-Phil