If you aren't playing arena, I'd probably say the pack quest, as that's guaranteed dust for 5 cards in exchange for 50 gold, rather than grinding for another 50 gold (15 more wins)...to buy a pack which might also be 5 dusted cards?
Of course, if you've no other packs than classic, you may as well go for 50 gold and start expanding your collection :)
Which one is more valuable if I already completed my classic collection ?
Packs are worth 100 gold so...
If you aren't playing arena, I'd probably say the pack quest, as that's guaranteed dust for 5 cards in exchange for 50 gold, rather than grinding for another 50 gold (15 more wins)...to buy a pack which might also be 5 dusted cards?
Of course, if you've no other packs than classic, you may as well go for 50 gold and start expanding your collection :)