For me, since I am in silver rank and I don't have enough cards to add up to my permanent power,
I always rent cards to gain the two daily chests rewards.
But if you don't aim the multiple daily chests rewards, then I recommend to get power at the end season.
But take note, renting cost is high at the end season.
Ye i wanna get the 2-3 daily quests everyday but renting power is pretty expensive. I only have 250 DEC and i need to use around 100 to get 40k power evreryday.