The 15-inch touch panel is only slightly higher-resolution than its smaller counterpart, at 3,240 x 2,160. Microsoft was clearly focused on maintaining pixel density (it's slightly less dense than the 13-inch model at 260PPI) rather than dazzling you with a 4K screen. It weighs more for both the combined system (4.2lbs versus 3.6lbs or less) and the tablet portion (1.8lbs versus 1.6lbs), for that matter.
However, you might not mind so much given the extra graphics power. The Core i7-equipped 13-inch model's GTX 1050 and 2GB of video RAM are respectable, but the 15-incher ships with a GTX 1060 and 6GB of video RAM. Microsoft claims the Surface Book 2 is fast enough to play games like Forza Motorsport 7 at a solid 60 frames per second in 1080p. While you wouldn't want to buy either laptop with gaming as the focus (you can clearly get more bang for your buck elsewhere), this makes the 15-inch rig a tempting option if you don't want to give up your game library just to try Microsoft's removable tablet design.
On that note, Microsoft has refined the system in mostly subtle ways. The Surface Book's signature hinge has been tweaked to wobble less when you touch the screen, while there are promises of a faster, more comfortable backlit keyboard. Newer processors help the machine last for up to 17 hours in laptop mode, and 5 hours in tablet mode. And finally, finally, Microsoft has deigned to include a USB-C port in a Surface device. There's just one on each model (you also have two USB-A ports and an SDXC card reader), but it'll spare you from using a dongle to charge your phone or attach a newer external drive.