Some ideas: Darting is typically a result of too much ski pressure or a mis-aligned front end.
Ski pressure: Back off the spring preload on the front suspension. When you do this, you will also get the feeling that the sled "pushes" in the corners. It's a trade off. Not too much you can do about it other than keeping good carbides on your skis.
Ski alignment: The standard set up for Polaris is to have the tips of the skis 1/4 inch wider than the tails when the suspension is in the typical riding position. Meaning flat surface and slightly compressed (NOT fully extended).
Once these two areas have been addressed to your satisfaction, spending money on skis comes into play. Best to go with skis that are specifically set up to mount to a Polaris. The spindle size is different on a Skidoo. I believe the main ski bolt on a Doo is bigger too.
Snow Goer did a write up on skis this last fall. Simmons came out on top. They are a dual carbide (on outside edges) design. Actually the original with this concept, that skidoo recently copied. I like SLP's, the original Ultralites are best, but the new SLT's are decent too. Finally, I have some tri-keels on a moutain sled. They're a good compromise between a powder ski and a trail ski. www.startinglineproducts.com