Which community edition branch to use?

We are using IDEA 10.5 ultimate edition to develop our language plugin.  We've been targeting the idea10 branch for development (we test the plugin by launching the binaries from the idea10 community edition).  We're at a point now where we'd like to use the plugin to develop the plugin :-)  The problem is that the idea10 based plugin is not compatible with IDEA 10.5.  I started using the master branch and fixed up the API breaks, but I'm wondering, should we use the master branch or does the x0x branch target 10.5?  Ideally I'd like our plugin to be compatible with both 10.0.x and 10.5, but it appears the API's have changed enough to make that problematic i.e., creating an intermediary API is probably a bridge too far. For now knowing which branch to use for 10.5 is enough to get us by.  Thanks.