A shared foundation
Fundamentally, the iPhone XS and the XS Max are the same phone -- one of them is just bigger. This itself is interesting, because in prior years, the iPhone 6 Plus and 6s Plus had cameras with optical image stabilization, while the 7 Plus and 8 Plus had dual cameras. The Max holds no such advantage this year -- instead, the two phones share a long list of tweaks and improvements worth noting.
Both, for instance, pack improved stereo speakers that offer some additional clarity and a more pronounced sense of space. This is especially apparent when you're watching movies or live music recordings -- it doesn't exactly feel like you're in the middle of the action, but the experience comes closer to that than I would've expected.
The glass covering the front and back of these new iPhones is tougher this year too, and that's a good thing. My iPhone X suffered a few serious gouges soon after I started using it, but so far the XS phones have withstood the hellscape that is my backpack. Even better, the XS and XS Max are (finally) rated IP68 for water and dust resistance, meaning you can leave one of these phones in as much as seven feet of water for up to 30 minutes. Not that you ever would. Right?
Both phones also have eSIMs tucked away inside, though it'll be a while before you can actually use them. The idea here is to offer dual SIM support -- so you can attach multiple lines of services to a single device -- without having to juggle two tiny pieces of plastic. (Unless you live in China, where regional versions of the XS make you do just that.)
This is a concession for international users who often have separate lines of service for voice and data, but Apple says you'll eventually be able to activate just the eSIM and skip traditional SIM cards entirely.
Lastly, in addition to the usual 64GB and 256GB storage options, you can now get an iPhone XS or an XS Max with a whopping 512GB of storage. That's not exactly unheard of -- Samsung also offers its massive Galaxy Note 9 with that much storage -- but it's a great, if expensive, way to make sure you'll never run out of room for your offline Netflix movies.