If you're looking for the fastest way to earn SPS, holding DEC is better than buying cards.

If you're looking for the fastest way to grow in the game, I would rent cards first, to get up in ranks and get significantly more DEC per win. (Rent Level 2 and Level 3 Commons to round out your teams). This will get you more DEC and will cost the least DEC. That'll get you more SPS

If you're looking to do BOTH at the same time, then you should definitely still rent cards, but yes, you should also buy cards to increase your power. Do something like 10% of DEC you win each day can go to Buying cards, 10% goes to Renting, and 80% you hold.

If one day you don't need to rent cards cuz you have a ton rented for the season, buy some with the extra 10%.

The benefit to getting to Level 2, or Level 3 Owned cards is that they'll help you win battles, they add to your collection power for airdrops, AND you can rent them out for some passive DEC each week/day if you end up not using them.