I don't have a lot of varied experience out West, but I've spent a week a couple of times at Vail and I've been to Mammoth many dozen times. Unfortunately I've never gotten to the Utah resorts as they sound great. I grew up skiing at Okemo and Killington. One thing east coast skiers learn is to use their edges which is totally lost on west coasters. The first time I skied out west at Mammoth there was this tiny quarter inch of crust (not ice) on stop of snow that was so easy to break through with an edge and get into amazing soft snow. It was like running a butter knife over the top of vary cold butter with a soft inside. It was about the best snow I had ever skied in and everyone around me was complaining about how icy it was. If you can ski anything at Killington outside of Outer Limits you will be one of the top 5% of skiers on a mountain out west. Another way to look at it, the worst day out west, assuming they have coverage in a non dry year, is better than 95% of the days you ski on the east coast. One other thing you can get out here is 'wind packed' snow on a cornice. Once again, west coasters will complain about how icy it is and you'll see them yard sailing it down the steeps because they don't know how to use an edge. Wind packed snow is my absolute favorite thing to ski on - you can carve turns like nothing you've ever experienced back east.
So my two cents: If there is fresh snow, the back bowls of Vail are the best thing I've ever skied. If it hasn't snowed in 5+ days, Vail isn't very good. Fresh snow at Mammoth is typically very heavy, hence the term Sierra Cement. It's still fun, but you have to be better than decent to deal with the 'powder'. I used to be a pretty good and was asked to join the Ski Patrol at a local Southern Cal resort and I have no problem with the cement, but most people I ski with who are good and can handle a black diamond OK, struggle in it. That being said, once the powder is gone, Mammoth takes care of their snow way better than Vail. 5 days with no snow and you get great corduroy runs in the morning and fluffy mounds as it gets moved around in the afternoon.
Hope that helps.