When I looked at the numbers last season it would take over 1000 days of rent to make up for purchasing a card. This led me to rent cards with my earnings to help improve my win percentage and allow me to move up in league levels. Rented cards count toward collection power for League requirements.

Buying the cards would count toward your daily SPS airdrop. But the payouts in Silver appear to be more than in Bronze. (~ 50% to 100% higher)