SMF 2.1 is not an official release yet or in other words its just in experimentation / development stage and not meant for live sites. In my experience its always best to wait til the software is well vetted before moving up because there will be bugs and security holes not yet known.
There is a new release for SMF 2.1 which they refer to as "SMF 2.1 Beta 3". I still would not put this on a live production site as there may be unknown security holes or bugs but at least it is good to see progress being made.
Simple Machines is proud to announce the release of the 3rd Beta of SMF 2.1. This version contains many improvements over Beta 2 and also fixes many bugs that were discovered following its release. Among the notable improvements:
Added reCAPTCHA as core feature to keep spammers away. Upgrade.php code clean-up. Now the upgrade process should be more consistent. RSS general improvements. Added support for jQuery 3.x. Modified Curve2 to improve the responsiveness of the design. General bug fixes, code improvements, and clean-up. Default codification will now be UTF-8. Source 6w