Leather is better!
When a new leather pump is installed, it should have a few drops of light machine oil applied before installation. And then use a sleeve to hold the leather in place during the installation so it doesn't turn upside down.
When an old leather pump is removed (if the leather has not turned upside down from poor installation) a few drops of light machine oil will generally rejuvenate the leather and it will still be superior to the artificial pumps.
Of course, even a leather accelerator pump won't turn the Rochester B into a real carburetor!