I don't have the Piston 3 but I have the VSD3(S).
They are quite impressive technically. I might get the pistons soon but it seems that the sound signature of both is quite similar. Controlled bass with a slight V shape. The VSD3S has quite a bit of treble energy so maybe the piston 3 sound more mellow?
I'm subbing this thread because I want to get other people's opinions too.