A good number of Arizonans will travel farther away for skiing over the holidays, so while Snowbowl will definitely be crowded, it might not be as crowded as the Summit County areas--particularly on weekdays. Still, if you pick Snowbowl, make sure to wake up early and get on the mountain early, as they'll just cut off access once the parking lots are full.
I'd say the green/blue/black markings inbounds are comparable to Copper and a bit easier than A-Basin, since you mention Summit County. Hart Prairie is an amazing spot for beginners to learn and gain confidence, and the new chairlift there should make things even smoother. It's really wide, with few obstacles, and even when crowded it's not as densely-packed as green runs at a lot of other areas. There's a real benefit from learning in a spot that allows you space to feel safe. The snowmaking water is not sewage... it's the runoff that goes down storm drains and, as mentioned, it's treated.
Weather and natural snowfall are unpredictable, so it's pretty tough to say how much of the terrain will be open. Summit County's more of a sure thing. If you really want to spend a week on skis, Snowbowl's a gamble... between the possibility of poor conditions and the limited amount of terrain that might leave you bored, Summit County's a safer bet. On the other hand, I'd argue that Northern Arizona has more, better, and easier-to-access nonskiing activities available if, for whatever reason, you decide to take some days off of skiing.
Are you and the rest of the group coming from Phoenix? Or somewhere relatively nearby? Flagstaff's a good deal if you don't have to pay for airfare... but if you're flying from somewhere else and renting a car, that eats up a bunch of potential savings.