For personal projects it’s a matter of preference. Grid is interesting but is fairly new. Bootstrap 4 actually uses Flexbox but does the work for you where that’s concerned.
When it comes to working for a company though, the framework you end up with is usually dictated by that company. Depending on where the company is you may be able to change that but it’s unlikely. You could end up with neither getting stuck with foundation or even a framework that the company built themselves (which is how Bootstrap started).
Grids and Flex are basic CSS. You’ll be a better dev for knowing them because you’ll have the foundation to do more. Knowing boostrap is just icing on the cake, and you can do more faster because you want have to start from scratch. Sure devs can get lazy and use out of the box bootstrap but you can push it much farther.
So like anything: learn the foundations, and you can adapt faster as your tools adapt. Frame works save time and sanity, so learning them is always a bonus.