The best shooter game on the PSP.
The main problem in RR is that the move of view is too slow. In DD, you can adjust it.
The level design of DD is good and varied. You can use different guns and gadgets, which offer players more strategies to complete the tasks. So you don't feel dull during playing it. This is not like RR, in which you only have one choice: finding cover - shooting - finding cover.
Oh, by the way, although having a short storyline, DD still immerse me in the game world. I even feel Gabe's rage. As Gabe, I wanna kill Touchstone and Singularity by myself.
SS:DD is much better than RR, though both produced by SCEA.