Assuming Q995B = Q990B and Q995C = Q990C in other region. The key technical differences is better SpaceFit which integrate both the AutoEQ and SpaceFit into 1 and also on the "C" model SpaceFit actually calibrate for subwoofer unlike the "B" older model that does not do that. And "C" has a much more refined audio tuning out of the box which tuned more closer to a high end home theater system. Example when set the "C" model to Adaptive profile, the sound frequency response it delivers without any additional tinkering is almost perfectly tuned to home theater standard. Lastly is an improve Q-Symphony if that matters to you as this only works on Samsung TVs. While the other features mentioned earlier is applicable to anyone with the soundbar.
All in all they are mostly software upgrades rather than hardware. Other than that, there are no significant hardware differences between the B and the C. You can simply say it's a refinement to the B model in every way apart from the hardware side of thing. That being said most of the refinements especially the tuning of the sound profile do have a pretty huge difference to justify going for the C model over the B unless they have big enough price gap that is actually worth the savings, i would say go with the C.
