Airdrop in the real sense of it should be easiest because of the fact that all you need do is just either indicate interest, fill or register through a form. However, many times airdrop coins don't get anywhere though some fly.
more reliable way to earn. If you are just starting to get acquainted with the world of crypto currency, then you can look at it. More advanced market participants can expect to earn a little more with the help of the Bounty
For me then, I don't prefer airdrops because they often times turn scam after your hopes have been put high. Campaign bounties are the best for me. You have a choice to choose which ico, also you know the pedigree of the campaign manager before going in. Bounty campaign is better for me.