The S Pen isn't the only difference.
Though very similar, the Galaxy Note 9's plus-sized battery, and the ability to take notes using the exceptional S Pen, make it worth the extra money over the Galaxy S9+.
A great, big phone: Samsung Galaxy Note 9 ($1000 at Amazon)
A Galaxy Note just wouldn't be right without the S Pen. Samsung's unique stylus has always been a highlight of the Note series, allowing users to jot down memos and draw with up to 4096 levels of pressure sensitivity. It's useful off the screen, too; hovering the S Pen over the display creates a floating cursor, which is great for quickly highlighting text and even scrolling through web pages, and the button on the S Pen opens a menu of shortcuts to various apps.
With the Note 9, the S Pen has been further improved with a supercapacitor that allows for remote use through Bluetooth. You can use the button on the S Pen to trigger functions from a distance, ranging from taking photos to controlling media playback, and much more.
The Galaxy S9+ already has a pretty sizable battery at 3500mAh, but it's no match for the Galaxy Note 9's larger 4000mAh cell. While 500mAh may not sound like much at first, the Note 9 has much better endurance than its Galaxy S counterpart, easily lasting through a full day and into the next.
Higher storage options
Another big reason to invest the Note 9 is its higher capacity options for internal storage. Sure, you can add a microSD card to either phone, but sometimes even that isn't enough to hold all of your data. There's still a $200 price between 128GB models of the S9+ and Note 9, but the former maxes out at 256GB for $860.
The Note 9, on the other hand, starts off at 128GB for $1000, and moves all the way up to a baffling 512GB of internal storage for $1250, doubling the maximum capacity of the S9+. Combine that with a microSD card and you can reach all the way up to a terabyte of storage in the palm of your hand. That storage upgrade also includes a bump from 6 to 8GB of RAM, for what it's worth, though you likely won't notice that increase in most situations…
A better DeX experience
… That is, unless you launch into DeX. While the majority of consumers may not care about this feature, Samsung has put a lot of focus on improving the DeX desktop experience on the Note 9. You don't have to buy a specific DeX dock anymore; plugging in any USB-C to HDMI adapter will launch the Note 9 into desktop mode.
You don't need peripherals like a keyboard or mouse anymore, either — though they're still good to have. The Note 9 can function as a trackpad for the desktop monitor, and you can even use the phone in mirrored mode or as a secondary display. DeX is becoming much more flexible and, in turn, more useful for more people.
A Great Big Phone
An incredible phone with a huge price to match.
The Galaxy S9+ is great, but the Note 9 is even greater. It does everything the GS9+ does, but adds in a larger screen, more storage, longer battery life and an S Pen stylus.