Learning Flexbox or CSS Grid in 2018 [on hold]

The next year is almost upon us and one of my goals is to put some time into improving my front-end design skills.

I'm trying to choose between the following to use for a small internal project:

• Flexbox
• CSS Grid
• Bootstrap 4 (I'm already very familiar with Bootstrap 3)

Hopefully I'm correct but from what I know, flexbox is in use and Bootstrap 4 uses it. CSS Grid is an emerging technology but compared to flexbox, there are pros and cons.

Any thoughts on what would be the most useful to know in the next year?