Part of the decision can be made based on what each site wants specifically. For example, some websites want only How-to articles, while others want articles about a specific location in the world. So those are good ways to gauge which articles go where. You can also determine where to post an article based on how revenue is generated. HubPages is fairly upfront with their policy: you take 60% and they take 40% (at least I think that was the ratio). Some websites don't disclose how they determine your payment and that can get discouraging after a while.
Another thing to consider is not spreading yourself too thin. While you might make more money writing for multiple websites, it is a lot of work. Writing for just one is a lot of work. So try to pick one site that you really believe in and focus on that. Then, maybe after you've generated a healthy backlog of articles, you could focus on another site for a while. Trying to do all at once just becomes frustrating and each individual site might be getting a half-hearted article as opposed to one site getting a bunch of really strong articles.