i am a noob but what i hear from a local sales person who sells both brand is this: soundstream amp has more impact (whatever that means... ), and PG amps is more musical (again, i don't know what that means).
I love PG, and have owned Soundstream in the past. But, I just got in my new VGA320.4 thanks to finding it at a heck of a lot cheaper price online than the TI500.4. I have heard that the Van Gogh series amps are the only worthwhile ones from Soundstream, but that is just from reading posts, not from actual experience.
the VanGogh line of amps most likely has won more accolades and awards than any other amp in the market today- high s/n 115db, high damping factor >500, and low thd-impressive specs- it was my amp before the one in my current sig