Big difference. Think of the down haul similar to your keel boat's back stay adjuster. It acts to depower the sail plan or power it up, depending on situation.

Be careful and spend the time to actually learn its capabilities. It is not an on or off thing. You can actually work the down haul instead of the mainsheet going up wind, depending on the boat and design.
Best to set the boat up on the beach or trailer and then watch the sail as you go from minimum tensions to full tension of the down haul. The body and top of the sail will likely change dramatically. Of course I am assuming a rotating mast on a typically stayed catamaran.

Good luck.