First I assume we are talkiing about free skiing. And the answer, as other have noted, is skiers hands down.
First, there aren't many skiiers out there who really go fast, or even like to go fast. You generally need racing skis to do this, and only a minority of skiers actually own these ( I would guess less than 1-2 % of all skiers generally). A vast majority of boarders don't use alpine carving boards either, which is the boarding equivalent.
You are better able to absorb bumps, etc, on any course whether closed or open on an independent suspension, ie. two legs. And you are always faster on a groomed run than on one with obstacles or powder. There is really no comparison to doing a downhill run on a closed course. It is the ultimate rush on skis.
Unfortunately at most resorts you really can't redline things on the groomers due to those moving speed bumps we affectionately refer to as intermediates. In order to seriously rip a run, I always check with the patrol first. Usually the best time to really run is early in the morning before the intermediates are out.
I've never seen a boarder going 45 to 50 mph, which is probably about as fast as anyone wants to go on a course which is not closed. Of course if you are plunking along at the average speed on a groomer (25 mph) you will see the occasional boarder passing you at 35 mph. But I've noticed a lot of boarders moving faster than 25 mph seem to have control problems, ie they can not check up to a safe speed as easily as someone on GS skis.
Next time you are on the hill pick out a skier who is on serious boards (GS or super G race stock) and see if anybody passes them.