Personally i would say Arma just because of the sense of scale alone. While it has some similarities to PR its also its own beast in its own right! Includes a huge amount of vehicles,weaponry and garments, this makes it great for setting up events and keeping them different, not to mention the bot support. The downside is people can take the milsim in Arma too the extreme.
Squad again another great game especially if you're looking for something a bit more updated to fill the spot that PR has left, however in its current state (optimisation) the lack of vehicles and such is a real hindrance. However i think the sights and sounds in squad are incredibly immersive, some very good maps to go with it.
Just a very brief overview, i own both and they are quite vastly different from each other, so if you have the funds i'd recommend getting them both (Arma is on sale ALOT)