Just switched this season:
Rails: Quicker to learn on a board (50/50s), but almost no learning curve when you change over; progress more quickly on skis
Switch landings: Skis are easier by far. I've pulled out switch landings on skis that I would have absolutely wrecked on a snowboard.
Actually riding switch: Snowboarding is way easier.
Jumps: Skiing is easier
Shifties / Grabs: Boarding is easier
Spins: Easier on a board (for me). I'm having trouble with spotting my landing too early and coming out of my spins too quickly.
Flips: Havent got there yet.
Things that have given me trouble after changing over: spinning, urban-on, reverts from switch at speed.
Overall I'd say snowboarding is easier to learn park on, but if you can already board then the learning curve to skis is pretty short.