You have to choose one based on own interests and various other factors like :

1) If the campaign is paying in its own coin, read about the project. See if the coin can be very valuable in future. Even if they have allocated less number of coins, you should go for it as the total value can be huge in future.

2) It is very important to join a genuine campaign only. Otherwise all your time and effort will get wasted. There are some way to judge the genuineness of a campaign like who is the campaign manger.