For investment you have to use your own money , and naturally will be very carefully and selective on which coins to buy and thereafter will pay more attention to monitor the market, and such  I think investment have better chance of making profit. By doing bounty campaigns and airdrop what you pay is your time and effort and not own money, so mentally tend to be less selective  as a result the free coins we get might not backed by prospective projects. Also because it's free most of us will not spend more time to monitor their price and would as a result miss cashing them at good price.