As with anything, it is highly subjective and greatly depends upon what one is looking for in a device. There are quite a few benefits to the Google Slate, some of which are simply not available on the iPad Pro. But if those benefits aren't something that are important to you, then you won't see the value in them.
The Google Slate is a tablet with physical dimensions that are comparable to the iPad Pro (12.9). But in addition to that it offers:
- a full desktop version of Chrome (not just a mobile version that can load the desktop version of a site)... this included support for extensions and plug-ins.
- system-wide support for trackpad/mouse. Support is optional... if you don't want to use it, touchscreen still works.
- near full support for USB peripherals. USB thumb drives, wireless keyboards/mice that require dongles, wired mice/keyboards, microphones, gamepads, even custom presentation remote controls.
- A system-wide accessible file system. Although chrome OS is optimized for the cloud, local storage is fully accessible.
- Support for Android, Linux, and Windows apps (via CrossOver)
I've spent quite a few years pushing the limits of my 12.9 iPad Pro to get it to cross the divide between tablet and notebook. It came pretty close but was still not where I needed it to be.
I recently bought an 8GB/128GB Google Pixelbook (new at Best Buy for $470 out the door). almost immediately, it went beyond where the iPad Pro took me and got close to that point of convergence.
I can see where the Google Slate could possibly be even better than that for me.
But having said that, not all things are rosey in ChromeOS-land.
The state of tablet-optimized Android apps is only slightly better than Modern UI apps for Windows. There's also not a quantity of quality apps... for the iPad (I've since replaced my 12.9 Pro with a 2018 iPad) there are a mountain of terrific notebook/notetaking apps... (I love GoodNotes!) on the Android side, only Squid comes close. That's what I use for my digital handwritten notes on my Pixelbook since it is optimized for the Google Pen.
In an attempt to be more flexible with my options, I try to standardize my productivity apps on Google's (they work well on the iPad, and even as web apps on my iMac), but when I need MS Office, I find that the online version of MS Office to be surprisingly well featured... far, far beyond the Android and iOS versions.
Chrome OS can't do everything (yet). Specifically video, graphics, and audio production... so for the foreseeable future, I'll be performing those tasks on my iMac.
When Chrome OS 70 is released (timed for the availability of the Slate) it is going to be even better for my purposes since when the Pixelbook is in tablet mode, the UI changes to a more iPad-esque user experience.
Chrome OS isn't for everyone, but I think that the greatest benefit of the Google Slate is to offer stiff competition for Apple to provide some of the above capabilities to the iPad Pro. If Apple were to introduce mouse, file-system, and peripheral support to the iPad Pro, it cause me to go back to the Pro as my primary mobile device.