The different weights of the BB’s are suitable for different situations.
The lighter the BB, the less accurate your shot will be. Use a heavier BB, and you can be more accurate, but at shorter distances. So, you need to decide whether accuracy or distance is more important to you in your particular airsoft game.
Also remember, that if you are playing with airsoft guns outdoors, the wind and other weather factors will influence a light weight BB, more than a heavier one. So, you must first consider where you are playing and then decide if accuracy or distance is more important to you before you load your magazine.
Another choice for you to consider, is using biodegradable BB’s. When you are playing outside and have no chance of cleaning up your BB’s, biodegradable makes a great option. They are a bit more expensive to buy, but you will feel better knowing that you are not destroying our environment.