I concur also with other post s:
Skymiles Gold or platinum is nearly nothing by AF, who prefers Frequence plus red, although red is easier to get for the one who flight transtalantic.

I do not think you will get an upgrade on national Delta flights, but you can get preboarding, etc, as a skyteam Elite plus member (refer to the Skyteam web site to see what they offer) as well as access to all lounges.

Preboarding is a reality by Delta, not by AF.