Biggest differences for me were more tracks/effects/synths in a project and the ability to have more than two simultaneous inputs while tracking. If you have't ran into either of these issues as constraints and MC6 is working for you then there isn't a compelling reason to upgrade.
HOWEVER
Unless the $70 for the upgrade (you are aware that if you buy this as a download/upgrade in the cakewalk store you get a significant discount) will hurt your budget, I thik that the additional plugs (Boost 11 in particular) would make upgrading to Essential a smart thing to do.
It is really worth watching the monthly specials in the Cakewalk Store. Occasionally they drop the upgrade price for Studio or Producer to a price point that you just can not reasonably refuse.
Glen