I own all 3 devices... well, except for the N7 that I returned. In my opinion the gushing praise for that device is unwarranted. It is praiseworthy, but doesn't rise to the level of "OMG! This is awesome!" that is common.
As an all-round device, the GTab 2 7.0 is a great choice. It may not have the same snap in performance as the KFHD or N7, but the support for microSD and available peripherals such as the keyboard dock, make it enjoyable to use.
Out-of-the-box, the KFHD is fairly limited. Usable, but limited. But when rooted, it makes a terrific Android tablet. Comparing the N7 and a rooted KFHD side-by-side, the KFHD wins out, in my opinion of course.
In my experience, the OS update capabilities for an Android device are not as important as they would be for other platforms, such as iOS. iOS devices generally speaking are on a "forced march" to upgrade... by requiring the latest version of the OS to obtain one of the "baked in" features/apps, or the nagging white-on-red "1" notification flag on system settings icon that stares out at you relentlessly pursuing you like Poe's telltale heart.
Having said all that, all 3 of those devices are fine. Even though I didn't keep the N7, it wasn't a bad device. Ultimately it comes down to personal preference and need.