Grid will be awesome very soon.
I think the argument these days should be “do X unless you’re unfortunate enough to care about IE”.
Css Grid is less intuitive but they do make grid gutters much much easier.
Most of the time I reach for flex box. Only when I'm doing a guttered grid would I reach for css grids.
With how hard it is to get a companies UI devs on the same technology and patterns, flexbox is an easy standard to coordinate around.
Not to mention it's better supported.
Also, if you're using styled-components (which I recommend), use styled-flex-component for layout and things get even better. It's such a breeze, it makes even complicated layouts so simple and fast to put together.