Perhaps my point is getting lost here.
What I'm referring to is not a change in generation, rather an offering within the same generation, of basically the same product(s) with virtually the same capabilities and pretty much within the same price-range.
At the high-end of the market, there's only a few, and they are similar to each other, with one only having a slight advantage over the other, but enough to make a difference, depending on the app/game (eg. GTX580 vs HD6970 or GTX590 vs HD6990).
In the mid to low range, this is a different picture completely..