In the last year and a half I've made 7 afghans for my family, with two more to go to finish out the roster!
I think when we, as knitters, get caught up in the beauty and textures of yarns we tend to think about how gorgeous a garment or blanket will be when it's made up and forget about the care it will need to keep it looking wonderful over the years.
Afghans are something that should give years of warmth and comfort and beauty, but if you use a yarn that could be easily or accidentally damaged the life-span of your wonderful work can get shortened considerably.
So, when you're planning your afghan, I'd suggest thinking about it in terms of living with it once it's finished. That's where the "art" moves into the "practical" arena. And that's what will make or break the longevity of the piece.
If you're doing wool, then I'd definitely consider a superwash variety. I've used Caron's Simply Soft acrylic to very good effect in several of the ones I've made and the recipients were incredibly happy with the soft feel and easy care of the finished piece.
But whatever you choose, good luck with it and let us know how it's going!