If you're noticing that much of a difference between regular and goofy, then goofy is for you. You do what is most comfortable for you, not what guidelines, charts, and rules of thumb tell you.
Skiing is worse on the knees inasmuch as the potential for catastrophic accidents. With a snowboard, having both legs attached to the same piece of wood ensures you have a minimal chance of jacking your knee - you tumble as a unit. If you've got skis on and one ski decides to catch and turn your leg 180 degrees.... well, your feet are designed to face the same direction, not one front and one back. I went X-country skiing last season and I lived in mortal fear that I'd torque a knee. I was so happy to get those damn death sticks off my feet at the end of the day. I'd sooner go X-country snowboarding next time.
Snowboarding gets much less strenuous on the knees once you learn proper form. Right now as a beginner, you're probably ruddering with your rear foot and muscling your board around. That will all go away once you gain confidence, speed, and proper form.
My advice is to just stick with what you ultimately plan on learning. If you want to be a boarder, skip the skis - send your ski gear back to the 80's where it belongs and stick with the board. If you could get down blues in one piece, you're halfway there already anyway.
If you want to ski, then skip the board.
It's just advice tho... take it with a grain of salt and do what you like.
"Never attribute to malice that which can be adequately explained by stupidity."
10/11 - 24, Smote on Feb 13