I recently bought a 49" X900E Sony and had to return it based on 1080/720 broadcasts looking too grainy and the display just being too bright for it's VA panel to not look washed out in a lot of bright scenes. Once you tune it so the bright scenes are more manageable, then the dark scenes have too much black crush. I mean even more black crush than my ISP panel TV does.
The 1080/720 broadcasts looking grainy on 4K TVs is a problem with virtually every 4K TV I've seen. Even the ones with good scalers and upconversion like Sony seem to only scale the content object based vs full screen. For instance in a football game close up shots of players will look pristine and clear, but the turf and background looks noticeably pixelated. Sky cam shots are the worst, players, turf, and stands all look pixelated. And it's problematic even in a lot of studio filmed TV shows where there's no fast motion. It's common that even Sony 4K TVs, with their always touting great skin tones, have problems with a lot of Caucasian faces looking pixelated on 4K sets with 1080/720 broadcasts.
On brightness, the X900E measures in the 500 to 550 range, but I find that is too much for an LED lit VA panel. It's contrast is in the 5500 to 6500 range. The X800E's brightness is in the 350-400 range, and it's also IPS with no local dimming. It's contrast is around 1000. I didn't find local dimming on the X900E to be worth using really. IMO the X800E 43" is the best gaming TV in the size you're looking for, but it will still have a problem displaying 1080/720 broadcasts as awell as a 1080p native res TV. If you ant to try it though, I recommend buying from Crutchfield, whom have a for any reason return period extended to Jan 25th on the holidays. The 43" model will also likely be priced much better than the current $650 a couple weeks before the Superbowl, which is by far the best time to buy a TV without risking it going out of stock.
I came away realizing there currently is no 4K TV that displays 1080/720 broadcasts acceptably, and I even feel the motion handling on sets that people claim is good, is really mediocre. The X900E gamed fine, but as an overall use TV, I can't recommend it. It's really touted mostly for HDR movies and gaming, because that's where it's high brightness and contrast make fuller use of HDR's extended color gamut. The tradeoffs are unacceptable though IMO. The 43" X800E would likely be much more acceptable overall due to it's smaller screen size and brightness and contrast that are more suited to overall use, vs just HDR.
I have decided to keep my 32" Panasonic IPS TV, to wait for ATSC 3.0 TVs, which should start hitting the US market early 2019. These current 4K TVs sorely need UHD content though, and ATSC 3.0 broadcasts will have that both over the air and via the net. It will be a hybrid broadcast system designed for everything from multi language options, to enhanced emergency broadcasts (can wake up a receiver in sleep mode), to UHD video with Dolby AC-4 audio, and far better reception with simultaneous multi device capability. ATSC 3.0 uses far less bandwidth than ATSC 1.0, can provide crystal clear reception even to fast moving cars in tunnels, and it's required equipment for broadcast stations is cheaper than what is being used for ATSC 1.0
ATSC 3.0 will not be a forced mandate by the government like ATSC 3.0. It will be totally voluntary and during the transition period there will be both ATSC 1.0 and ATSC 3.0 broadcasts, with many options for early adopters of ATSC 3.0 from sets that have the new tuners, to addon ATSC 3.0 tuner boxes, and even small USB type tuners. This is not a conversion scenario like it was with analog to digital though. Both ATSC 1.0 and 3.0 are digital and can be transmitted through existing RF towers (with slight modifications).
Sorry for the long winded diversion to TV broadcasts, but I really feel too many people buying 4K TVs are unaware of ATSC 3.0, and can benefit from learning more about it. Here's a great interview conducted by one of the AVS forum staff with Richard Chernock, whom is on the panel of design engineers for ATSC 3.0. ATSC 3.0's design should be finalized by early next year. The only part of it I strongly disagree with is when Chernock implies current TV scalers, being a lot better now, are good enough to scale HD to UHD. IMO that is far from the truth, they've gotten better, yes, but not THAT good. Maybe it's an agreement industry wide to just blatantly imply that so people will be more inclined to early adopt 4K to pave the way for UHD broadcasts. If you recall the main criteria for making HD broadcasts happen was the number of homes having HD capable TVs.