Crystal may not be a resort as compared to Whistler or Park City, but it is a destination. What I mean by that is once you are there, all your needs are met.(as in no more need to drive)
I believe that all of the places mentioned (Stevens, Crystal and Snoqualmie) have excellent beginner terrain with very good grooming. The instruction is as good as you can get at all of them as well. I think that the on-snow beginner experience will be pretty much the same (Snoqualmie is a little more beginner friendly) When you consider snow conditions, I don't think that one is better than the other either. They all get unpredictable PNW weather. I had rain last March at Stevens, epic deep powder in November at Crystal, and ice in February at Snoqualmie. You get what you get. Crystal has higher elevation, but Stevens can get slightly better storm patterns sometimes. They will all have nice snow on the groomer runs. Views are still unbeatable at Crystal (if it's not cloudy)
My main consideration is the apres-ski. I would rather ski to the door, and be drinking a beer while throwing a steak on the grill, than dealing with getting all my stuff back to the car and driving back to the hotel through a snow storm. After the BBQ.... a little swim in the 98 degree pool, then throw a log on the fire while watching a movie and a hot buttered rum. All of the condos at Silver Skis are private and furnished with a kitchen (you can bring your own food to cook). Some have games or books and a nice stereo and DVD player. In the morning, I would rather sleep in, make a cup of coffee and then wander up to the lift lines for first tracks, than negotiate the snow storm, ice on the road, traffic, parking lot, schlepping gear, shuttle bus, locker room ....etc. (It's actually not that bad once you get used to it, I do it every weekend)
You are on vacation after all.
If you still want to do Stevens, consider renting a cabin. We stayed in a nice one called Mysty Mountain just 15 miles west of the pass. It had a hot tub. see http://vacationrentalcabins.com/
Actually you could stay in Seattle. If you don't mind driving. It's only about 10 more minutes or so drive from Seattle to Snoqualmie, than Stevens to Leavenworth. Then you would have all the night life you could want. Speaking of Seattle, Here's a partial list to check out: Music - Benaroya Hall, Triple door, Dimitriou's Jazz Alley, the Moore, The Paramount, the Showbox, the Tractor Tavern, the Nector, The Crocodile. Food - Ray's boathouse, Sun Ya for dim sum, Village Sushi, Santa Fe cafe, Zeek's Pizza, Metropolitan Grill, Buenos Aires Grill, Theo Chocolate, Zig Zag Cafe, Bakery Nouveau... There's tons more....
Whatever you decide have a Happy Birthday.