Played them both for quite a bit of time.
Hell let loose is much better looking, the environments are really great (because they are scanned from real locations), but the game is still pretty poorly optimised. I also find the player base hardly ever works together, and there isnt much tactics involved other than putting down/hunting garrisons and staying on/attacking the obj with bodies. No transport vehicles, so getting around the map can be a bit of a slog if your officer doesn't put down spawn points, or at the start of a match.
Post Scriptum is much slower paced, doesn't look as good with much more empty maps (when compared to HLL) and is a bit more clunky/janky - but the team focus is where this game shines. You cant win by just 'throwing bodies' at an objective - having and using a microphone is generally required by most squads although as long as you can just listen and follow instructions, yo should be fine without talking)
Depending on what style of game you like, I recommend trying both and refunding what you don't enjoy.
Hell let loose has a free weekend this week as well.