Hello... I wish you luck in making the right decision and offer my view.
A few years ago, I was solving the same question - and I chose Delta.
It was a bit too difficult to figure out how the Flying Blue program works, which booking classes earn what etc., and Delta was straightforward and clear from the beginning.

I have an EU address. I usually fly AF or KLM between Europe and USA two or three times a year and book the flight on AF or KLM web site even though it is in DL metal. That way I earn 100 % MQMs even for the cheapest tickets toward DL status (as opposed to booking with Delta and earn nothing for E class).

Despite flying mostly on cheap economy tickets to earn the status (= free bag + seat selection), the most important thing that made my decision for Dela was that Delta miles do not expire. Not that it is worth stacking them up, but...
In addition, I also earn miles towards Delta Airlines MIllion Milers, which earns a permanent Sky Team status one day.

Silver status is enough for me: with it, I can use the short Business Class check-in line, which saved me time at the airport several times when the lines were long.
And I use credit card with Lounge Key in order to get to the lounges.

Today, I see that Flying Blue does clearly say which booking class earns what:
https://www.flyingblue.com/en/earn-s...elta-air-lines

And also I can see that even Delta Basic Economy (E) class earns 20 %.
BUT it still is not at all clear to me how many XPs I would earn for flights with partners in order to reach FB status.
So, simly put, I just do not choose Flying Blue for my lack of understanding it.

PS: A nice new update with Delta is that I can earn miles even for flying on award ticket. I am not sure if this is also true with FB.