The Surface Laptop is a smaller, more compact device no matter which way you look at it.
The Book's rounded hinge creates a gap when the laptop is closed, which causes it to have a tapered profile. The entry-level Surface Book is 13 mm at its thinnest point; the Surface Book with Performance Base (the more souped-up model) is 15 mm. Both max out at 22.8 mm at their widest point.
The entry level Surface Book weighs 1.51 kg (3.33 lb); the Performance Base model weighs 1.65 kg (3.64 lb). That's about 17-percent and 24-percent heavier than the Surface Laptop, respectively.
Here's a key difference: The Surface Laptop has the standard clamshell form factor that we expect from a laptop. The Book is a 2-in-1 tablet/laptop. The keyboard bundled with the device detaches completely, so if you'd like, you can use the Book as a giant tablet.
Build material (case)
Microsoft uses a magnesium alloy casing on the Book. The Laptop is aluminum.
The Laptop goes all-in on color, with four different options available. Silver is your only choice for the Book.
Screen size and aspect ratio stands pat.
Both devices have touchscreen displays, not just the tablet-like Book. They're also both compatible with the Surface Pen stylus.
The Surface Laptop's 2,256 x 1,504 resolution is not shabby at all, but the older Book still beats it with a 22-percent greater pixel density.
The keyboard on the Surface Laptop has an Alcantara fabric covering. This gives it a soft, luxurious touch, but it might be prone to greater wear and tear over time. The keyboard on the Surface Book is a more standard plastic variety. Both are backlit for easier typing in the dark.
Neither machine has a fingerprint sensor for password-free logins.
...but you're not entirely out of luck on biometrics. Both the Laptop and the Book support Windows Hello's secure facial recognition, letting you log in just by showing your face.
Surface pen included
While both devices support the Surface Pen stylus, only the Surface Book comes bundled with it.
We're still waiting for clock speeds for the forthcoming Surface Laptop, but its 7th-generation Kaby Lake processors should represent a boost over the Book's 6th-generation Skylake chips. In the Book, the i5 clocks in at 2.4 up to 3.0 GHz, while the i7 is 2.6 up to 3.4 GHz.
The Surface Laptop uses embedded graphics, but with the Surface Book you have the option of upgrading to NVIDIA GeForce graphics.
The entry-level Laptop starts with 4 GB of RAM and is also available with 8 or 16 GB. The Surface Book starts off with 8 GB and maxes out at 16 GB.
The Book is available in four storage sizes ranging from 128 GB to 1 TB, but the entry-level model is not available in the largest size and the Performance Base edition is not available in the smallest.
The Laptop keeps things simple: Select 128, 256 or 512 GB of storage. Microsoft VP of Devices Panos Panoy mentioned a 1 TB option at the Surface Laptop's launch event, but we've yet to see it reflected on the Microsoft product page. Perhaps it will become available in the future.
The Laptop has one USB port; the Book has two. Both of them stick with legacy USB 3.0 ports instead of the newer USB-C standard. They also both sport Mini DisplayPorts.
SD card reader
Photographers will probably appreciate the Surface Book's built-in SD card slot, which is missing from the Laptop.
You won't have to occupy one of those precious USB ports to charge the device. Both have a Surface Connect port for dedicated charging.
While the headphone jack is disappearing from some smartphones (ahem, iPhone 7) it's still a fixture on tablets and laptops like these.
Both devices support Dolby Audio Premium, which should signify some measure of sound quality, but they have different speaker configurations. The Surface Book has a front-facing set of stereo speakers while the Laptop has more adventurous "omnisonic" speakers that are actually underneath the keyboard.
Microsoft promises an impressive 14.5-hour maximum battery life on the Laptop. Of course, manufacturers' estimates are to be taken with a grain of salt, but we will be impressed if it meets those expectations in practice.
The Surface Book also has staying power, but keep in mind these estimates are with the keyboard accessory attached. The Book's battery is split between a larger one in the keyboard and a smaller secondary battery in the tablet. If you use the Book without the keyboard, battery expectancy drops to a few hours.
However, it is possible to detach the keyboard and reattach it in the opposite direction. You can still tap into the keyboard's battery while the board itself is tucked behind it in stand mode.
The Laptop has a 720p webcam, but the Book has a tablet-style camera configuration with an 8 MP rear camera and 5 MP front (selfie) camera.
The Surface Laptop is the first announced device to sport the Windows 10 S operating system. It looks and acts much like the full version of Windows 10 with one huge exception: It only runs apps from the Windows Store. It's like Microsoft's answer to Chrome OS, a walled operating system targeted at the education market.
It is possible to upgrade to a full version of Windows 10 if the S version won't be enough. That upgrade is free until the end of 2017, but after that, it will reportedly cost US$50. On the other hand, the Book comes with Windows 10 Pro from the start.
The Book came first. The standard model launched in October 2015. The Performance Base edition came out a year later. The Surface Laptop is available for pre-order now to start shipping and hitting shelves June 15.
The price difference here is significant, especially considering the overlap in specs and features. The entry-level Surface Laptop, which is lighter and smaller, starts off US$500 cheaper than the most basic Surface Book, even with a full version of Windows 10 (if you opt in soon). Still, if you go the Laptop route, you'll miss out on perks like the flexibility of a 2-in-1, higher resolution display, an extra USB port and options for improved graphics.