Why do you have to choose if you can do both? 
Airdrops are easy to do, because it requires very small amount of effort. Though, remember that small efforts is equals to small rewards.
Bounties requires effort to be done, because of the tasks and deadlines.
If you're doing Airdrops, you can do Bounties too or vise versa.
Don't limit yourself!
I think bounty campaigns are better than airdrops in terms of token earned, in bounty campaign a participant has to promote and advertised the project through social media and this requires more effort than airdrops but in a long run their effort will paid off if the project was successful and also there is opportunity to join many social media campaigns of bounty to increase the amount of token to be earned.Airdrops are easy to do, because it requires very small amount of effort. Though, remember that small efforts is equals to small rewards.
Bounties requires effort to be done, because of the tasks and deadlines.
If you're doing Airdrops, you can do Bounties too or vise versa.
Don't limit yourself!