I will recommend to begin customizing in CRM 2011 Online trial version and create a solution where you can export and import on CRM 2013 Online trial version later on.
For customizations ensure that if you are using JavaScript use the XRM.page objects commonly instead of using XML Dom to traverse the hierarchy, also there is a conversion tool that will identify the issues on JavaScript code.
CRM 2013 will have other features that will not required JavaScript like field Business Rules, that you can customize using the UI. helping with development
I don't know how soon you will get the move to CRM 2013 how ever it's been well documented that it will have blackguard compatibility with CRM 2011