Depends on how you like to play Hearthstone.
If you like one class and you play only with that one, you should buy card packs. You lose time by playing arena and also if you don’t win any game in arena, you waste gold because one pack of cards is 100 gold while an arena entry pass is 150 gold.
Not winning any arena game rewards with a pack of cards, around 15-20 gold and around 20-30 arcane dust.
So, playing with one class = buy card packs.
Don’t care about classes = play arena.