These are also the two players I'm currently looking at. I popped into the local Sony Centre last weekend and took a look at the X800M2. I really liked the look of this machine. It's definitely well built and at £265 a bit cheaper (the Panny doesn't seem to drop below £299).
The lack of auto switching between Dolby Vision and HDR10, although slightly annoying, isn't, that big of an issue, at least IMHO. What is a concern is the numerous accounts of disc sticking issues on the Sony which the UB820 seems to be free of.
But as two2midnight pointed out some don't have any issues with the Sony. I'm really on the fence regarding the choice between these two players. Until I can make my mind up I'll just stick with my Series X.
My current tv is the LG C1 which I picked up about a week ago.