The good news is that both places will do exactly what you are looking for - great mountain getaways. Estes Park is located at the East entrance to Rocky Mountain National Park - probably the BEST mountain scenery in all of Colorado, maybe the best in the nation. Lots of easy hikes on well groomed trails - and some more difficult ones if you want that too. Great opportunities to see lots of wildlife in the Park. Estes Park has a lot of places to eat, but I would not say it has 'high end' lodging. The Stanley Hotel is likely your only option. It has a lot of branded hotels, just not high end. The Stanley is famous for being the hotel that inspired Steven King to write The Shining - and that is cool. But there are crowds at the park - especially on the weekends, so be prepared for that.
Vail, and the other ski resorts in the area (Keystone, Breckenridge, Beaver Creek, and Copper Mountain) will all have high end accommodations. In August it will be quiet and you have the mountains right out your door. The resorts also have lots of activities (at least Keystone and Breckenridge for sure) for families like paddle boats, alpine slides, bike riding, mountain biking, hiking (but not as good as Rocky Mt Natl Park), fly fishing, golfing, Dillon Reservoir, and activities in the evenings. You also have the option of outlet shopping in Silverthorne, which is close by, or white water rafting in Glenwood Springs, which is about an hour west of Vail. If you are more adventurous, there are several 14ers - mountain peaks with a summit over 14,000 feet - close by that you can climb.
Either way bring layers of clothing - it gets chilly in the evenings even in August and be prepared for summer showers in the afternoon. They are short, but can be intense. Also, save your strenuous activities toward the end of the vacation after you have acclimated to the altitude somewhat. Going up a flight of stairs quickly can be a challenge when you first get here from lower altitudes. And - you will hear this constantly - drink LOTS of water. It tastes pretty good and will keep help keep the altitude sickness at bay.
My recommendation would be to do the ski resorts for the majority of the trip and the return to Denver via Rocky Mountain National Park from the West Entrance. It will take the best part of a day to drive over the mountain and make all the stops you want to make, but it is well worth it. It is literally unforgettable. Whatever you do - Welcome to Colorado and Have a Great Vacation!
