The basic difference between Flexbox and the CSS Grid Layout module is that flexbox is designed for 1 dimensional layout, that is laying things out as a row OR a column. Grid is designed for 2 dimensional layout which means columns and rows at the same time.
Rather than repeat myself, I have a blog post and code on this very issue which you can find here rachelandrew.co.uk/archives/2016/03/30/should-i-u…
However I'm not sure whether by "traditional CSS grid layouts" you were thinking about a float based layout. The difference between flexbox and floats tends to be most apparent if you want to align things, which is what flexbox will give you. You need to take some care with flexbox that you don't get the content shifting issue that Jake describes here jakearchibald.com/2014/dont-use-flexbox-for-page-…
Until we have grid layout in browsers then I think most of us will content to use a mixture of float based grids and flexbox, and just test to make sure that what we are doing works well on slow connections and also with older browsers. I don't have a "decision checklist" because every project is different. You have to look at the audience for the site you are working on and the design you need to implement and make those choices on a case by case basis.