Gems for buying packs, keys for playing against legendary duelists at the gate (more keys = more chances to farm high level LD's for their specific cards), coins for card trader and specific cards
Packs of 10 are about 500 gems I think, I have like 5000 or more cause I'm trying to conserve n see how people play so I can spend on packs I actually need. I basically hoard gems the most. Cards for card traders usually range in the 1000's, I have about 200000. Keys I have about 200 each and still stocking up, I don't farm LD's yet cause I'm f2p and deck isn't good enough yet. Ah well, longevity!