I will say that, after nearly 16 years of playing Project Reality, and maybe 2 or 3 years of Squad. I can only say that Project Reality offers a far greater amount of content and variance compared to Squad.
If you want to experience a game similar to PR that has the eye candy most are privy to these days, Squad is good. I like Squad, but personally, until they catch up or exceed the amount of maps, assets, factions, weapons, IR imaging, and.. God forbid.. Proper CAS. PR will always hold a greater value, especially considering it's free, still regularly receives updates, and has a dedicated community attached to it like a seimese twin.
Alas, I'm biased and taking into consideration Project Reality has been a near daily routine in my life, for almost half my life, it will always hold a special place in my heart. A lot of great, fond memories that I will bring with me to the grave.
Oh, and despite my... Surly temperament, I have made more than a few lasting friendships, here at VG and else where on PR. I'm proud to call it my home away from home, so to speak.
EDIT: I feel the need to clarify the statement above, what I mean is, Squad isn't as tightly-knit as PR. In the sense that, it simply hasn't been around long enough to develope a cult-like following like PR has amassed. PR is from a different time. A simpler time.