I dislike thinking about stuff like this. It's a waste of time and a distraction from things that truly matter, e.g., Pluto! But, a bunch of jerks already set the stage and the tune and it behooves those of us wanting to profit on that stage to be aware.
If you have data or citations to back up the "most common and easiest to understand" please share, I'm always interested in seeing data. Here's what I have to offer, for what it's worth:
https://books.google.com/ngrams/graph?content=man-hour&year_...
The problem is that an ugly little debate exists outside of this thread. You don't actually get to drag in small pieces of pedantry like that in isolation.
So, on the topic of this debate:
There are serious real problems in the world and in the software industry with people who are jerks, and (in particular, topically) unwelcoming and offensive to women. This is a very bad thing. And there are people who wish to fix this by changing the language we use. That's a legitimate position to propose and advance.
It's also a legitimate position to object to. But there are those come into the discussion wielding the hammer of Moral Authority, and often they suggest that if you fail to join the crusade to change a word like "man-hours" then you're an ignorant bigot riding high on all your privilege (you can already find some posts on this thread suggesting ideas from this group). Really, it isn't strictly about treating women decently, it's about demanding your subscription to and endorsement of a particular worldview and series of political prescriptions... it's about securing power and finding ways to stifle your opponents and detractors.
A POX ON BOTH YOUR HOUSES.
You're being mentally lazy for no good reason.
Literally nobody commented on the actual content of the OP.
That's what I mean by defining the default. The default, in your point of view, is the word "man," so therefore anyone who objects to that must be "different" in some way: overly sensitive, manipulative, deficient, etc.
When in reality, "man" is the default because men made it the default. That doesn't mean that it has to remain the default.
I'm not suggesting that we go back and edit Neil Armstrong's words--they were of their time and are amazing. I'm suggesting that we're in a different time now, and starting now, it is within our power to choose the words we use.
The idea is to recognize the history and context of language, not assign all problems to one word. No single drop thinks it is responsible for the flood, etc.