Both Vail and Aspen are great. You can't go wrong with a summer vacation in the mountains of Colorado. Aspen and Vail top the list of best places to be. Which is "better" is very subjective and depends on the goal of your vacation. Either one will have great dining, shoping, activities (rafting, biking, gondola rides, fishing, etc). Both areas also have great culture (Vail hosts Bravo! Vail Valley and Prima - find details at vail.com). Vail has more/better golf courses.
Again, both resorts are great. You are certainly not making a "mistake" by visiting one of the top resort towns in ski country (and summer is the secret season - you'll love it).
As part of the commuting workforce, I have spent a good deal of time in Aspen. My trips to Vail were just weekend trips for pleasure. As the previous post said, it depends on what you are looking for. Both offer good nightlife, dining and shopping. I think Vail has a few more shops geared towards a moderate budget. I also enjoy the walking in Vail more, perhaps because you don't have to deal with traffic. Both have weekend markets, but Vail's is larger. When stay in Vail, it is very easy to take a day trip to the Breckenridge/Frisco/Dillon/Silverthorne area, if you have a car. You could also stroll around Beaver Creek, Avon & Edwards by taking the bus or a short drive. I love Aspen, but I think you will find more to do around the Vail area.
I personally prefer Aspen (big surprise) as I don't enjoy the interstate whizzing through the middle of Vail.
At this point I'd suggest you search both the Aspen and Vail forums as this was a wildly popular question last summer and I think you'll find a lot of additional information that will help you make up your mind. Good luck!
Thank you all for the advice.
When you are in the Villages, you never even know the highway is there. Everything on the north side of the hwy is mostly residential, so with the exception of grocery stores, Sports Authority, fast food and gas stations, you don't spend much time on that side of the hwy. Most people either love Aspen, don't care for Vail or love Vail and don't care for Aspen. I happen to love the both for different reasons. Regardless of your end choice, you could always do a day trip to the other just to see what it is like. It is about a 2 hour drive from one to the other.
You will not go wrong with either. Anyway that you could do both?
A bit partial to Vail but I could easily be persuaded differently.
Like others have said, No worries either way.
My vote would be for Aspen, but not having been to either, I think you'll have a great time no matter where you choose!
The biggest plus to vail is proximity to the airport and any other mountain towns you might want to visit.
Both areas offer alot. Vail is easier to get to. 100 miles straight from Denver on I-70
And as a comment to the previous poster about I-70 whizzing "through" our town> unlike Aspen where you have to constantly dodge all the traffic troughout thier town and fight for parking spaces> we keep our traffic out on the interstate and have a peaceful and safe "pedestrian only" village! ;-D