A penny sleeve is loose by design. Slide a card in without lining up every edge, sort a stack of 200 in twenty minutes, done. A perfect-fit sleeve is the opposite: snug enough to sit like a second skin, made to go inside a toploader or a sealable box rather than stand alone.
Penny sleeve alone
Fine for bulk, for a deck you're actually playing with, for anything not worth a second thought if it gets a light scuff. It stops dust and fingerprints, not much else.
Perfect-fit alone
Better surface protection than a penny sleeve because it doesn't let the card shift inside it, but on its own it still doesn't stop a bent corner. It's an inner layer, not a replacement for rigid protection.
Both together
This is what actually protects a card worth protecting: perfect-fit sleeve directly on the card, then a toploader over that. The perfect-fit stops the card from grinding against the toploader's inside edge every time it's handled or shipped, which is where most surface wear actually comes from, not from being bare.