Having flown behind a 796 that was hard wired and mounted on my panel, if your eyes will tolerate it, the 660 is a better in a few ways,
The 660 screen is brighter.
Even though the 796 is touch screen it doesn't work that well(old technology). You have to touch it very positively in flight, not like a iPad or iPhone.
However the 660 touch feature is just like an iPhone or iPad.
The 796 has XM weather and music capability.
Currently the 660 does no not have XM capability, however I think by the end of 2017 there will be a antenna that you will be able to bluetooth to it to have XM.
They just dropped the price on the 660 to $749, darn I paid $800 at OSH 2016.
The 796 is about $2000 on Amazon.
I betcha that XM antenna for the 660 will be $300.
The 660 is way easier to update. Just hook to wifi and it will find the update if it needs one. McDonalds here I come! Sweet!
The 796 has to be hooked to a computer via USB.
The menus in the 660 are similar to the newer GTN 650 and 750 Navigators so when you upgrade you will feel right at home.
But in the end its all up to you the pilot. I suggest checking them out at a show near you like OSH 2017 or Sun & Fun coming in April.