Sentinel vs PAM first?

Hi everyone. I'm fairly new to DnD - this is my first campaign and we just hit level 4. I've heard the PAM/Sentinel combo is wild and my party suggested i go for it. Currently i have GWM and at lvl 4 now, I'm not sure if I should get PAM or Sentinel first. I'm a Path of the Giant barbarian with a Halberd so have 15 ft range. I've heard on some forums that range makes sentinel worse and it procs less. That said, besides myself, my party is a grave cleric, a warlock, a sorc/monk, a rogue, and a fighter, so I'm the only tanky one. PAM seems like it would be more versitile, as i can stand out of range and throw something at an enemy, and they have to come into my range in order to get to me or run past (so i get at least 2 attacks on them before they even get past me), where with Sentinel, i have to get right up next to them in order to get the reaction, and they have a lot of room to run around me before triggering my OA. I was even thinking of just getting PAM at 4 and taking the ASI at 8 and maybe not getting sentiel at all as i've also heard it's not as good higher levels.

But again, I'm new and don't really know what I'm talking about. Thoughts?