Stay on 2.7 - at least for the moment.
Get familiar with thingy and its settings.
Wait before upgrading, there have been reports of bricked devices when updating lately, see:
It seems that a "safer" procedure is to upgrade SwOS from within the booted Ros (rather complex), but it is not wholly confirmed:
Bricked CRS328-24P-4S+RM after SwOS upgrade - #10 by petertosh
If you want to upgrade, 2.16 should be OK, but don't take this statement as guarantee of any kind.
In any case - again for the moment - do not upgrade Ros to higher than 7.19.6 (better be safe than sorry, though I haven't seen reports of briocked devices, it seems like 7.20.x has some issues in functionality, particularly using Webfig/browser).