They are two different chains, no need to choose between them. As long as you're logged into both your post can be sent out simultaneously. I'm guessing they'd be published at the same time since both have 3 second block times. Dtube displays them as one post for example and when you log in to the chains separately you'll see that chain's version of the post.

As for different people posting the same thing on the two chains. That would simply mean there would be two posts of the same thing on different chains. :P Which would mean you'd see them both on dapps supporting Steem and Hive.