This will essentially boil down to "Come to my mountain, it's the best!". I'll do the same thing, but I'll give you a whole lot of reasons why. I would suggest checking out East Coast resorts as well as those out west. The pros of an East Coast trip:
1) A much shorter flight from the UK. The price is likely to be roughly the same, between 500-600pounds. However, the flight time to Denver is about 10 hours out of Heathrow, whereas into Boston or Montreal, it is more like 7-7.5 hours. That's if you get a direct to Denver, which is a scarce commodity. If you have a stop, you're likely to be in transit for up to 16 hours. Direct flights to the American Northeast (or eastern Canada) abound. I'd suggest flying into Boston if you chose a New Hampshire or Maine resort, Montreal if you want to go to Vermont.
2) The terrain is more amenable to beginners in general. The Rockies out west are "young" mountains. That means high elevations, steep slopes, much more severe terrain. The Appalachians of the east coast are "old" mountains, much more worn down and weathered. This means much lower elevations, more shallow slopes, and generally a kinder, gentler mountain range. Advantages: lower base and summit elevations (between 300-600m base elevations on average) mean no acclimation time. Smaller mountains are by their nature less intimidating. Staring up at a 3500m+ mountain is much less likely to inspire confidence than looking up at a 1200m mountain that may or may not break the treeline. Now, that doesn't mean there isn't challenging terrain in the East. There are some mountains which offer a variety of everything, and will keep the best skiers happy.
3) Should things go awry, there are more alternate options for your wife if you do an east coast trip. From the mountains in Vermont, Burlington VT is easily accessible, as is Montreal. Boston and New York are not too far away from any eastern resort, either.
I'm sure I could keep going. However, I figure this is enough to be going on with. Now, there are a number of areas on the East Coast to choose from. Anything south of New England and New York tend to be small day areas. Your destination resorts are generally in northern New York, Vermont, New Hampshire, and Maine. There are also some mountains in Quebec which you could choose. I'll speak to my area of knowledge, which are the New England resorts (Vermont, New Hampshire, Maine). I'll zero in on a few resorts, and tell you why I've winnowed out the others.
Maine- A couple larger resorts (Sugarloaf, Sunday River). However, a very long haul from any international airport.
New Hampshire- Some good resorts. Waterville Valley and Loon have some good terrain, and are solid mountains for all comers. North Conway NH is nicknamed "Ski Town USA", and for good reason. Attitash, Cranmore, and Wildcat are all within easy distance. I'd give some serious consideration to a mountain in that area. Attitash and Wildcat are good areas. Cranmore is smaller, more tame.
Southern Vermont- A number of major resorts (Killington, Okemo, Stratton, Mt Snow). However, with the exception of Killington, the terrain at Southern Vermont resorts tends to be rather tame throughout. Great for the wife, but not so great for the rest of the family who may want to do some serious skiing. Also, Southern Vermont resorts tend to get very crowded from their relative proximity to New York and Boston (particularly Killington).
Northern Vermont- Also a number of major resorts. Relatively less crowded. Jay Peak, Smuggler's Notch, Stowe, and Sugarbush are all worth a look. Of all of them, Stowe is the largest and has the most varied terrain. The village of Stowe also offers the most developed ski town of the bunch as well. Jay is a little more remote to get to. Smuggs' prices are a little better, but their infrastructure is old (no high speed lifts). Also relatively limited lodging options, and little to no life outside the ski day. Sugarbush is a great mountain. Good terrain, okay ski town.
So at long last, Stowe is my recommendation. I have spent 25 years skiing throughout New England. When presented with the choice of moving anywhere and making any mountain my home area, I chose Stowe. Stowe's amenities are excellent. They have a brand new lodge slopeside. They have an entire mountain which is mostly dedicated to novice and intermediate terrain. That's for your wife. For you and the kids, there is some very challenging terrain. Stowe's Front Four trails are legendary. There is also some great sidecountry skiing, and awesome backcountry opportunities off of the Chin and Nose of Mt. Mansfield.
Also, you are looking for a good instructor for your wife. I happen to be rather familiar with the Stowe Ski School. You see, I happen to work as an instructor there. I'd offer my services, but I'm only part time there, so chances are I wouldn't be available for a week at any point. However, BushwackerinPA is a full timer there, and an excellent instructor. Epic also instructs there. They can both give you a great rundown of full time instructors, and who would be the best fit for your wife, whether it is them or somebody else.
There's a great deal of food for thought. Best of luck with your choices.