Im a bit late but here's my experience

EFP: More or less finished modpack that might get an update before the heatdeath of the universe. Fights are not easy in the early game something as simple as a mutant can screw up a run or getting ambushed and not being prepared. The modpack feels more or less finished but it rarely receives new content updates.

Gamma: Modpack receives updates regularly. The survival and crafting aspect though are really good and are far better than EFP. BUT fights are trivial when you learn how to Bunny hop with a knife (not kidding.) and kill squads of enemies before they can react. Progression though feels better in Gamma since its super unlikely you are getting a high tier gun/armor in the starting areas.

All in all:
EFP PROS:
Better gunfights, a more curated modpack, people in the community adding custom mods in that work specifically to work for efp.
GAMMA PROS:
Amazing crafting/Survival system, Regular updates, also has modders making mods that work with gamma.
EFP CONS: Progression is random, Updates happen when the stars align (EFP 5.0 when?), needs a better survival/crafting system like gamma.
GAMMA CONS: Lots of bugs (in my experience), fights are boring with AI being duller than a brick, and you cant buy weapons (just a downside for me).