Suggest to have a group of radio buttons or a combo-box in preferences page with options Stable, RC, Beta, and Nightly. Stable will be the default choice, and when user switches to another selection, e.g. RC, we will scan the list of available update sites and replace the ones containing /stable with urls containing /rc. So for the base Studio, http://preview.appcelerator.com/appcelerator/studio/standalone/update/stable/ with be disabled and http://preview.appcelerator.com/appcelerator/studio/standalone/update/rc/ will be added and enabled.
This will apply to the add-on plugins that we provide as well, and we will keep the update url naming convention consistent across all the features. It will help reduce the likelihood of having incompatible versions of plugins installed against each other.
We could also have a remote file that contains the list of our update url prefixes, and we will append the right update stream type for the full URL to add and remove. This avoids the issue of user modifying the list of update sites manually and we no longer able to find the original url to replace.