Airdrop is simple and easier way to earn some free coins, but most of these coins are not worth anything. On the other hand bounties can give you a chance to earn some coins of good projects which can turn out to be huge in future. Though you will have to work diligently for a month or two for completing the tasks of bounty but at the end you would have earned some coins with real value. Having said that, bounty is a choice & commitment and airdrop does not take more than few minutes. So, I sign up for all airdrops (or most of them) and chose the bounty for the projects I like.