The Gnosis can be a better booster, but that isn’t always the case.
Porp mining foreman burst is stronger though, about 10.9% more yield per boosted ship.
The Gnosis itself mines about as much as 94% of a Venture.
As a result, if you have a fleet of 9+ Ventures/Prospects huffing the cloud, the Porpoise is in fact a better booster.
But even with a smaller fleet the Porpoise brings utility that the Gnosis doesn’t, like a much bigger ore bay, ability to take frigate holes and remote repairs.