When comparing Slate vs Backblaze, the Slant community recommends Backblaze for most people. In the question“What are the best power user tools for macOS?” Backblaze is ranked 10th while Slate is ranked 21st. The most important reason people chose Backblaze is:
The default configuration includes everything except for system files to be backed up. That also includes external USB drives. This will work for most people. It can, of course, be customized to each person's liking.
Specs
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Pros
Pro Extremely configurable
Slate is similar to Linux WMs like Xmonad in that it is nearly infinitely configurable. You can either use the default configuration file or just Javascript for very complex/dynamic settings. Slate also offers editing a ".js" or a ".slate" file for configuration.
Pro Based on hotkeys
Hotkeys can be set to re-size and focus windows, as well as activate layout presets.
Pro It's free
Slate is completely free to download and use, which is a great option in a field of Mac WMs that often costs money to use.
Pro Allows for tiling
Slate allows for customizing the config file in order to emulate tiling windows.
Pro Lets you set default layouts
By using a feature called "snapshots", users can set up different default window layouts in Slate that they can switch to and from on the fly.
Pro Window hints
Slate offers window hints which are an intuitive way to change window focus.
Pro Beginner-friendly defaults
The default configuration includes everything except for system files to be backed up. That also includes external USB drives. This will work for most people. It can, of course, be customized to each person's liking.
Pro No file size or traffic restrictions
There are no limitations of file size or amount of data you can upload. The $5/mo plan gets you unlimited cloud storage.
Pro Excellent restoration performance
Backblaze creates a zip file of all your files for you to download, thus decreasing the amount of data you need to download and increasing the speed of the download since it's one connection instead of multiple that need to be opened and closed for a list of files.
Pro Secure
AES encrypted files are transferred over a secure SSL connection.
Pro Download any backed-up files from any web browser
Some backup services like Carbonite require a client to be installed to restore files. Backblaze allows you to login and download any or all files from a web browser, meaning it also serves as an ad-hoc cloud storage platform.
Pro Users can order a physical copy of their data
You can have a flash drive or an external hard drive delivered to you. They cost $99 and $189 respectively.
Cons
Con Project is no longer in development
Development is no longer active on this project as of 2017.
Con Hard to configure
There's no graphical user interface for configuring Slate, meaning it has to be done from the command line interface or by editing a config file. This makes it somewhat difficult and time-consuming.
Con Missing features
Unable to perform all tasks of software it was meant to replace.
Can not move windows between 'Spaces' (virtual desktops).
Con Video fail
Demonstration video failed to highlight any of the advantages of the windowing system. Or at least, the advantages were so subtly intrinsic that no-one who didn't already use the system could appreciate them.

Con No Linux support
There's no Linux client for Backblaze.
Con Pay extra to keep deleted files permanently
Additional $2/Month plus $0.005/GB/Month for files updated, changed, or deleted more than one year ago.
Con Backblaze does not support network (NAS) drives
If you use a thing like Drobo, Backblaze will not back it up.
Con Buggy
