both are exact same game.
just that one u buy the license key then download the game, while another u get the installation disc with key.

diff is only the price. key is usually cheaper, maybe by 10-15% chepaer than the disc.

buying keys from random web store are somehow risky. so only buy from reputable sites.

P/S: if u feel that buying from bliz store is not safe, then u should only get the disc from local retailers.

G2A is having discount on weekends, keep an eye on that.