I have grown both and they are similar. Both are 100% indica...at least the ones I have dealt with. I really like the taste of Kush. Very smooth and falvorful. I could smoke Kush just for the taste.
The Afghan is a more stronger stone. It is sweet as well but it also has more of a skunk taste to it. Both are very good but I prefer the Afghan, especially when I need to get slammed to the couch! I also like the ease of growing the afghan. It is a true "plug & play" strain. The Afghan is also the better producer. Both clone very easy so thats a toss up. The Kush is a faster grower than the Afghan. The Afghan is the slowest plant I grow.
Well, I hope I cleared that up for ya! LOL. I am blazed at the moment so my mind and my fingers are not on the same page.