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.