What Legally said about transit-friendly is going to be the determining factor for you. For Seymour, you have to pay the Seymour shuttle ($6 each way) from Parkgate Village to the mountain. For Cypress, there's a Greyhound-style shuttle from Lonsdale Quay, which is $23 round-trip. With Grouse, you can take public transit right to the base of the gondola.
Several of my friends have Y2Play passes at Grouse, and they are unanimous that going during the day on weekends is a waste of time. You'll spend all your time standing in the line-up. Get there for opening, ski for a few hours, go home. Come back around dinner.
The runs, compared to Seymour, are longer but are generally flat (side-to-side), wide, and featureless. Very difficult to find side hits and such. Grouse has much harder black-diamond runs than Seymour, with a ton of moguls. They have a better beginner park than Seymour, but I don't think they put as much effort into the intermediate/advanced stuff.
If you're taking transit, this won't affect you, but if you drive and park, the gondola means you have to take everything up with you that you'll need for the day. Sure you can go down the gondola, go to your car, then go back up, but that's a lot of time and effort.
The Y2Play is a bit more expensive than the Seymour pass (if you buy in September). If you really want to do Seymour, you can buy a season shuttle pass which is about $160 IIRC. It may more or less work out as a wash (I buy family pass, so if you're buying an individual pass that may not be true). Depending on how far you have to go, this could make the commute unreasonably long. However, the bus that goes up to Grouse kind of meanders through North Van, so it's not a lot of difference in total time between Grouse and Seymour.