With a Premium in the name, the latest Sony flagship leaves no doubts what market segment it's competing in. The Xperia XZ2 Premium is also priced accordingly and if you're eyeing up Sony's 4K smartphone you've already allocated a significant sum towards your next phone's purchase.
That same sum (or within 10% in either direction, depending on where in the world you reside) will get you a Galaxy Note9, and we're not saying it'll be better spent there, but yes, it will probably be better spent there. The Note will bring you substantially better battery life, overall superior display save for the lower pixel density, and a telephoto camera that we consider more useful than a monochrome one. Then there's the headphone jack, twice (heck, even possibly 8 times) the storage, and also the S Pen for what it's worth.
If you are fixated on that monochrome camera after all, Huawei's got you covered with the P20 Pro. Thanks to some triple camera action, the P20 Pro can produce some great looking 3x and decent 5x zoom shots, so if you're into zooming in, the Huawei's the better choice. Battery life is also in the P20's favor, and you're also looking at some savings if you opt for the Chinese phone.
Speaking of savings, for the XZ2 Premium's asking price you could have almost two OnePlus 6s. Sure, that would mean a useless secondary camera, lower-res display, no microSD slot, and no meaningful dust and water protection, but also potentially more built-in storage, ultra-fast charging, and arguably one of the best software experiences on the market. Also, to reiterate - massive savings.
On the other hand, if money is no object (which it probably isn't, since you're contemplating the XZ2 Premium), the iPhone X is a viable option too. Again, that would mean a telephoto vs. monochrome camera dilemma. Against the super sharp 4K display of the Premium, the iPhone X puts up one of the best AMOLEDs on the market. With battery life being similar between the two, the OS could help settle it.
Sony's finally come up with a dual camera, and the high-res color module plus the high-sensitivity monochrome one makes for a versatile setup that puts out pretty great images. The 4K display remains unique to Xperia Premiums some three years since the first one, so it probably is a bit overkill, though it does make for nice headlines.
We'd have loved to see better battery life out of the XZ2 Premium, and we sure would've preferred a less unwieldy handset. Or at least of the two, because a device this big with just okayish battery life doesn't make all the sense in the world.
In the end, in the absolute highest market segment, there are both better all-rounders for the same price or more affordable devices that don't necessarily perform worse than this Xperia. If you're a Sony loyalist looking for the best the company has to offer, the XZ2 Premium is the way to go. If you're the more reasonable type, there are likely more sensible options.