I don’t see the difference.
Maybe the confusion is with “pay on PUT”? Perhaps clearer as: you must pay in order to PUT.
The way you do this is by paying to credit your account with an amount of storage that is then used up by uploading, storing messages etc (each PUT will shave a bit off what is left). Once exhausted, you need to “top up” again.
This avoids the problem with the cost of a single PUT being less than a Safecoin, and similar issues.
Well, that’s my understanding! 