I have heard the 6100 and the Energy Take Classics, can't say I have heard the C-50 or C-100 or the Fluances. Personally I wasn't impressed with the 6100 at all. I mean they work ok, but Onkyo is an electronics maker, not a speaker maker. I thought the Take Classics were better than the Onkyos, but that is subjective. I didn't hear them in the same store in a similar environment either though.
I personally would lean toward the C-50 or C-100 due to being larger speakers. They are going to have a wider range of sound. If you have a larger space to fill this also helps with that. You can probably keep your Fluance rears/sides if they are fine. Side/rear speakers don't need to be excellent speakers. You could also keep your sub, if you are happy with it. Or get a non-energy sub. The front 3 + sub are the most important though, and the front 3 should match.
As you seem to be looking for a smaller set, maybe a smaller set would be better for you. The Energy line is pretty well received from a few posts I have glanced at. I don't think you would be disappointed.