I would choose airdrop over the bounty. But good projects are very rare who is doing airdrop. last few years I joined airdrop for just 2 projects Dfinity & Polymath. You can work on bounties too. First research a good project with good potential in the market. then join their Signature & Translation campaign Also joining the telegram campaign is easy as well.

Though I personally think Bounty & airdrop both are a waste of time. The best way to earn bitcoin is by providing services. First, think about what you're good at, then come up with an idea then provide services & take payment in crypto.