I think knowing the movie heavily takes from Michael Herr's "Dispatches" also helps. It plagerizes the shizzle out of it. Good book and they lifted some of the true stories for scenes. They took Heart of Darkness for the main story and than pasted moments from Dispatches all over it.
Personally I think Apocolypse Now wins for entertainment value, but has little to no historical value other then those moments being legit. (like the M79 guy at night, that really happened at Khe Sanh, Herr writes about it, and they put it in the movie.) Still it's so quotable, and funny and weird..it's just a fun ride.
Platoon is more credible, and accurate over all but also takes some rare incidents that sadly did happen, and makes it seem like that was happening all over the place, so kinda crappy way to treat a lot of Veterans. Who never were a part of all that.. but I don't know what Stone personally experienced, so maybe he really was surrounded by POS guys who knows. Many of his buddies stuck by him so maybe that went through some really bad units.
Personally I prefer We Were Soldiers to both of those movies.