Title
If I understand it right gun slinger speeds up your throwing animation so the ball will get out faster and help you avoid sacks, set feet lead speeds up velocity of throws meaning ball will get to your receiver faster and you can fit in more tight windows. both are very good and serve different rolls so if you can get both
If you can get both for free, pass lead elite over set feet lead, as it's the same thing but works no matter what.
Besides that, what the guy above me said.
Assuming that these or standard abilities, it depends on your playing style, and to an extent your opponent’s scheme. You can throw into tighter windows with Set Feet Lead since the ball travels so much faster. Gunslinger can help if you tend to hold the ball longer, which naturally leads the larger passing windows.
As an X-Factor, Set Feet Lead is the choice, as you aren’t having an issue getting the ball out if you’re able to complete consecutive passes downfield.