From SharePoint 2013, we can create a site collection for both new UI and old UI of SharePoint 2010. If you have more experience from SharePoint 2010, you should create a new site collection by select 2010 in dropdown list, and if you need to spend a time work live with new UI, by default SharePoint selected 2013 UI.
Today, I create 2 site collections, 1 from 2013 and another from 2010 to make some checking on my web part UI functionalities. Visual Studio 2012 lets you create new project from SharePoint 2013 template including Apps and Projects. Download here
However, after finish developing the code and deploy solution to SharePoint, it’s only a feature from SharePoint 2013 UI in Site Collection Feature even thought the scope of feature is to be farm or site. Because we often forget to consider about package item in project solution and thus we don’t know exactly what about the place where SharePoint 2013 will install our solution.
Just open solutions package (from xml file) as a editor, we can see the property named “SharePointProductVersion” and change the value as you want.
For SharePoint 2013, you could change to “15.0″ as a valued and ”14.0″ for SharePoint 2010. If you need to be available for both 2013 and 2010, please remove this property and re-deploy again. Take it easy baby!