A great post. As a regular skier in Colorado, I support the points made, adding the following:

1) Suggest avoiding the US holiday periods, including Presidents Day weekend (and most of the rest of the week) as the resorts can be hideously busy - this can coincide with UK half term so check the dates.
2) Breckenridge is a great base (where I normally go) and the expansion in the area in the last few years has increased the skiing options. It is also a good base from which to explore other resorts - Keystone, Vail and Beaver Creek can be accessed on the same lift ticket.