Hi Philip,
I don't think there's a simple answer to this question, concerning that it depends on what are you interested to do. It's better if you can split your interests in two different areas: new features or stability.
If you want the latest features, such as all the work being done with XoWiki and XoTcl in general, and also the accessibility improvements for dotLRN, you should go to the latest stable version, wich is oacs-5-4 right now. However, if you try this option, you better have a good knowledge about the platform, because there's always somethig that's not working the way it should, and you'll have to fix or improve it.
If you focus is stability you should go for the stable version from the last year, wich oacs-5-3 right now. In this case there are some new things you can't use, but the Core and the tools available have less chances to crash.
I don't think this thought is shared by everyone in the community, since the stable version should be really stable, but that's what I've seen with my experience using it. I usually download the new version and spend sometime trying it in my environment before I go with it in production.
Make yourself comfortable to ask more questions in this matter if you want it.
