I write for both.
I like to write hubs and on Yahoo.
It is easier to earn more money by direct payments from Yahoo and their Pay per Impression model means that if you generate a lot of traffic to your pages you can generate a regular but small income.
HubPages for me has better community support and I get more tools to write and drive traffic.
Yahoo does allow you to write on any topic, but they do not always pay up front for the work, especially if it is on a common topic of interest. But they give you options where you can post an article to them which pays you per impression and you can adapt that piece of work and use it for a hub if you like.
I'd say give writing for both a good six month to one year tril if you can. Then see how it goes, if you feel that it is not working you can drop one or the other.