I have the Sony VPL VW665ES.
Some claim the JVC has a slightly brighter picture, but lacks Sony's superior upscale and motion enhancement quality. Sony is also better suited for gaming due to it's superior lag time. The JVC also has an 18Gbps chip set that allows 4K at 10 and 12 bit 60 fps. Sony does not, but the reality is there is no plans for content expect experimental at those formats now. The film/ motion picture industry is years away from those formats, stuck in 8 bit 24 fps and that is supported just fine with the Sony.
Many will look at price and go for the Epson or other low cost projector. I had an Epson and the big thing it lacks is the color processing. These are great for presentation but they lack the state of the art natural color processing in the middle priced projectors. Also, be careful about comparing MSRP because there is no need to over pay. I paid just under $8600 for my 665ES in an auction. The JVC RS600 and the Sony 665ES sell for about the same street price or about $9000-$10,000 In the US Sony publishes a really high markup for it's dealers but some will discount heavily. JVC, not so much. The dealers don't have much to play with so you get stuck paying near MSRP.
Going very high end seems to buy you new technology like laser light and brighter image for larger screens, but now were in the $60,000 MSRP range.
The Samsung gets along fine with the 665ES for BT709 color gamut, but doesn't cooperate at all for BT2020 + HDR which is what most of the UHD 4K disks use. I had to make custom color adjustments to get a good picture. Supposedly Sony has a firmware fix to be released later this month to fix the problem.
tigerfan is right, the current state of the art with these UHD disks is a crap shoot and really early adopter problematic.
BUT- the Samsung Netflix and Amazon apps with 4K content is amazing! Both have a bunch of great content now in 4K. The quality is really great. Plus the Sony's Reality Creation upscaler addes a 4K look to most of the local TV stations, and any 1080i output from my Dish 722K. The difference between a regular HD TV in 1080 mode and the 4K from the Dish 722K is day and night.