one bar to go
I carried STASS for a number of years in my previous job and it is an excellent system, yes, you must breathe out on the way to the surface to avoid the risk of embolism.
It also has the considerable advantage that you don't need to have it in your mouth, armed and ready when you enter the water.
With a re-breather system you can blow all the water out of the hose that runs from the bag, if the mouthpiece wasn't in your mouth at water entry BUT ONLY if you are the right way up. I have tried to use the system inverted with it out of my mouth at water entry and you cannot get the water out of the hose. I am assured by the divers at the dunker that even they cannot blow the water out.
So lets take the circumstance of a controlled, or uncontrolled, ditching where the aircraft rapidly inverts after entering the water and before you can put the re-breather in your mouth and arm it. The re-breather is only going to help if you don't mind swallowing all the sea-water in the hose, and that's quite a lot. STASS will function no matter what attitude you are in when you put it in your mouth.
I would strongly suggest that if you are flying the aircraft you are unlikely to have had the time available to the passengers to sort out your re-breather before the aircraft enters the water. I don't work for submersible systems but for me there is no contest, STASS is the better system.
If you don't believe the above try it next visit to the dunker, or ask the divers they will confirm it.