There are far too many variables for anyone to predict which way might work out cheaper. I have flown quite a few times with long terms between flights such as you describe. What I did was buy return tickets that allowed me to change the return for little or no penalty in cost. When something is that far in advance, I always want to make sure I can change the return if necessary.

I don't try to find the cheapest fare. I recognize that given the way airlines price tickets in real time these days using 'dynamic pricing', it is impossible to predict when to buy to get the lowest fare. It doesn't matter if it is a one way or a return in 2 weeks, never mind months apart. I do some research to try and determine the price range the route I want to travel tends to sell for. If the cost of two one-ways or one return varies between say $1000 and $3000, then I decide how much I am willing to pay. If I decide I am willing to pay $1500, then I look for a booking at that price. Once I find one, I book regardless of how far in advance and regardless of whether it is one way tickets or returns.