I chose the Death Star over the Executor for several reasons. The display footprint for the Death Star is smaller than that of the SSD. I simply don't have the space to display the SSD properly. I like the Death Star for its sheer variety. It's got a lot of features, moving parts, and attention to detail. Comes with a great set of figures, too. It functions equally well as both a playset AND a display piece. It's a great conversation piecefor hardcore Star Wars fans and even casual ones.
For me, the SSD is primarily a showpiece and has very little play value. As great a ship as it is in the movies, I actually prefer the design of the LEGO SD over the LEGO SSD. Considering they are both the same price (US $400), I think the Death Star is a better value than the SSD.
/my two cents.