I used ~25 spins and only got one but someone in my clan got 4 from ~16 spins. I'd say they're definitely rarer from the small sample of people in my clan who also reported what they didn't get. Its unfortunate I had 50+ saved but wasted on COS (and I ended up not even needing the resources).
Got all the masks from the previous batch though, so combined I now have like 8 free teleports to the slayer tower daily.
Not like Slayer rings do the same, there's a fairy ring nearby, loadstone nearly and canifis teleport.