I have no experience with the 120, but it is a different animal -- a filter -- and not a purifier.
As mentioned, when I tested out the Sawyer purifier at home -- after priming with the supplied rubber cup attached to my kitchen faucet -- water flow was quite good. But out in the field -- because I had failed to prime prior to my trip -- the output was absolutely miserable.
After my trip, when talking to Sawyer customer service, the lady was emphatic that the purifier must be primed with a faucet every time it dries up (i.e. when stored between use). When asked, she even said the unit will NOT prime properly by sucking or manually pushing through a bladder of water. It needed the force of a faucet, so she said.
I didn't much like the idea of starting every trip with a wet filter/purifier -- so I quickly sold mine. I did not, however, try to disprove the Sawyer lady by sucking on the purifier. Maybe a current owner of a Sawyer 125 purifier can give that a try and report back?