When to use line icons and when filled icons ? Is it something related to personal choice or it depends on the visual balance of the screen.
I think it's important to consider the visual balance while choosing they type of icons - plus the cognitive load as well. My personal opinion is to use solid icons as they reduce cognitive load and increase interaction speed.
Here's a great study done on this topic:
Averaged across all 20 icons, hollow icons were selected about 0.1 second slower than solid icons, which would seem to support Johnson’s assertion that hollow icons require more cognitive effort to recognize than solid icons (if only a very small amount more). However, the picture isn’t yet complete.