We also had a great time on the Southwest Splendors (Grand Canyon) trip. It was pricey - but one of the perks was that you stayed in the modest hotel right on the rim of the Canyon. Some people were disappointed with the hotel, but it was part of what made the trip fantastic - getting up at sunrise, staying out at sunset, it just couldn't be beat. I think it is hard to get a reservation there if you are doing this trip on your own.
I'm not as convinced that
ABD is necessary for domestic trips vs. Europe, but if I were going with my kids and no other adult, I'd definitely do a group trip - one of the very nice things about it is that not only do the kids get to make friends, but you do too, and you don't have to worry about anything. If you can afford it, you should give it a try - you'll have a great time.