Bump STR to 18 or GWM? 2014 5E

Hi! I'll try to keep it short, I'm playing a vhuman (PAM) fighter that I intend to take into Rune Knight. I wanted to ask what's better, to bump STR to 18 or grab GWM at level 4?

For reference, my party consists of a Bard, Barbarian, Sorcerer, Rogue, Cleric, two Wizards, and a Ranger (I know, we're a lot of us.)