In my opinion, satin gives a more "realistic" look, with of course a light weathering.
When you Google "Sterling l2a3", most of them seem to be in a kind of satin finish, or something between flat and satin. But glossy ? No way !
(And it depends on the armor going with it too... I guess if you ask here that the blaster will go with a TD armor, so yes, flat black should be better; damages by the sand, burnt etc... But for a TK, satin would probably look better !)
But of course,, you should paint it like you prefer