My biggest problem with Prometheus was that the “aliens” did not seem to be of the same species as the “A L I E N”s. It seemed like a completely different story, and it threw the whole thing off for me. I am given to understand that Covenant explains this. Not sure I’m willing to pay full price to find out.
There is a very good reason why the zenomorphs in Alien, are not the same. Chronologically Alien takes place 27 years AFTER the event's depicted in Prometheus and 17 years AFTER Alien Covenant.
These two prequels to Alien COMBINED set the stage for Alien. And Alien Covenant does indeed answer THE question which you and so many asked and wondered about for so many years: Where did they come from and why. Ridley Scott does a masterful, yet subtle job of moving forward the narrative he so aptly began with Alien and "tying up (most of the) 'loose ends'".
Alien Covenant is worth seeing on the big screen. But read the two reviews I cited AFTER you watch Alien Covenant and then rewatch Prometheus... Scott puts all of the main story arc pieces of the puzzle together and it's frighteningly well done.
Prometheus and Alien Covenant are thinking man's science fiction that deal with profound themes and issues and Ridley Scott is a master story teller.