I chose the 8 over the 10 for a few reasons. I prefer the smooth upper range of the smaller driver. I think that it will integrate better with the planar tweeter. (just my guess) To load properly, the 10 needs a box big enough to house a rottweiler.
Both are very nice drivers. They just have very different box requirements. When I modeled the box, I pretty much got the results I wanted with the alpha 8.