I like Late War if only because I like to fly the Nikki and they're only available there. That, plus more people in LW so easier to find a fight.
What I do is, if LW Orange has more people than Blue I'll go there, unless there's TOO many people. If one has 180 and the other has 120, I'll take the one with 120. If one has 17 and the other has 112, I'll go with the 112.
I realize that doesn't always help with the lopsided numbers but one time I was given a message by AH that the higher numbered arena was full and that I should go to the other one that had less than twenty people in it. I obliged and had a good time regardless of the fewer players.