I think the biggest difference is the power consumption R9 290 consumes much more power than a GTX 970. During BD playback specifically, the 290X consumes nearly 80W while the 970 consumes 12W. It's not just that AMD's cards consume more power in gaming/heavy loads, but they also consume more power where you wouldn't expect it--multi-monitor idle and media playback. AMD cards tend to upclock and upvolt for these tasks even though it may not be necessary (?) while Nvidia cards stay at/near idle specifications.
It seems like AMD could keep lower power states for these tasks and not have problems but perhaps they are overkilling to prevent any issues at all? Can't really tell for sure unless you're willing to edit BIOSes and stuff. The only time power consumption on AMD cards seems to be particularly good is in ZeroCore idle, lol. (And note you will never ZeroCore if you're using one or more HDMI displays.)
As for image quality, they're pretty much all the same. Traditionally AMD is usually slightly ahead in that dept. but it's such a non-issue for most people (and the difference is so small between Nvidia, AMD, and Intel) that there's rarely a review on it. Here's one here from 5 years ago, comparing an HD 5870 to a GTX 480 with HQV: https://www.techpowerup.com/reviews/HQV/HQV_2.0/8.html
I honestly wouldn't worry too much about the quality differences, if any. Honestly the GTX 970 is much better for an HTPC, IMO. Much less power (and heat) and usually less noise as a result. Prices seem similar these days too, so I would go with Nvidia (and this is coming from someone who has almost always owned ATi/AMD cards and still has all AMD cards at present time).
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