This really all boils down to what you and your family is looking for in a TV. You did hit the nail on the head with the differences you mentioned, but the C8000 also offer a higher Dynamic Contrast Ratio (8,000,000:1, vs the C6900's 7,000,000:1). Although the numbers are off by a million, that million contrast does make a difference in the colors and picture quality. The C6900 actually also offers a feature called HotApps that is not available on the C8000 (which instead offers Yahoo! Widgets along with Samsung Apps). HotApps is a Best Buy exclusive feature only found on the C6900 and C670 that adds additional applications to your Samsung Apps library (CinemaNow and Napster to name a few).

My personal opinion on 3D is that it is not a fad at all. All the manufacturers are pushing very hard to get this technology into their consumer's homes. With cinema going the way of 3D, more and more content will be shown in a 3D format. You already have cable and satellite channels that are 3D, and you have several Blu-Rays that will be coming out in the near future that offer 3D content.

I understand that currently, there may be a lack of 3D content, but since the Samsung 3D TVs offer adjustable 2D to 3D conversion, you can easily convert your 2D broadcast into 3D. That may be something to consider.