It's up to you and depends what features you need and if your environment allows you to install the latest version (I know some organizations prefer to stay one version behind, such as many government agencies).
From the latest information I had received, you can only purchase CF 2016 (CF 12) at this time. To downgrade you must purchase CF 2016 via the Adobe Volume Licensing Program. It appears that you can purchase a single license for downgrade through the volume license program as per a comment by Elishia on this blog post: http://blogs.coldfusion.com/post.cfm/adobe-coldfusion-backward-licensing
"No, a single license can be backward licensed if it is purchased through our volume programs. If it is purchased from the online shopping cart it wouldn't qualify. Please feel free to email us if you need to discuss the possible scenarios."
(I know some organizations prefer to stay one version behind, such as many government agencies).
I wouldn't say "prefer" to stay one version behind. Quite often, it takes forever to get anything through testbed. And to complicate matters, once something has passed testbed there are usually about 5-7 updates that have been released since the software started testbed, and they want you to patch it and re-submit for testing. GRRRRRR!!!
And if Adobe has stopped issuing licenses for CF11, I would not be one bit surprised. Mo' money, mo' money, mo' money!!!
I'll chime in too - there are dozens of issues (including some regressions) that are slated to be fixed with the as-yet-unreleased first update for ColdFusion 2016, with at least 120 issues still "open". At least until then, you might consider holding off on trying to deploying a production application to ColdFusion 2016. But as Nic said, if your application doesn't use any of the features touched by these issues, you may be okay.