When comparing SizeUp vs aText, the Slant community recommends aText for most people. In the question“What are the best power user tools for macOS?” aText is ranked 49th while SizeUp is ranked 57th.
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Pros

Pro Keyboard centric
SizeUp has user-configurable keyboard shortcuts for a set of pre-defined actions.
Pro Supports multiple monitors / spaces
Windows can be sent to different monitors and spaces. It can also be aligned as needed.

Pro Simple
SizeUp is easy and simple to use without having to configure.
Pro AppleScript support allows extending functionality
AppleScript permits adding features such as tiling all open windows. Additionally, FastScripts it allows the user to add keyboard shortcuts to AppleScripts.
Pro Menubar dropdown for shortcuts
Learning a bunch of keyboard shortcuts right off the bat may be difficult to remember at first, which is why a menubar shortcut for the app exists with all the shortcuts listed.
Pro No Update
Pro A snippet can be expanded to plain or styled text and include pictures
Pro Has built-in snippets for common typos
Pro Cheaper than competition
$4.99, cheaper than textExpander ($35)
Pro Compatible with TextExpander snippets
TextExpander snippets can be imported into aText
Pro Works with VMs
Pro Imports from most alternative text expansion apps
aText can import data from TextExpander, TypeIt4Me, SpellCatcherX, Automaton and CSV file.
Pro Has common HTML & JS snippets built-in
Pro Snippets can include editable fields
Pro Data can be synced through cloud storage services
Pro Snippets can include a wide variety of variables
Possible variables include date, time, other snippets, clipboard content, etc.
Cons
Con Nag window
If a license is not purchased there is a nag window (which suspends function) that pops up when loading the app (at user login). The nag pop-up is also summoned at certain time intervals and by performing a certain number of commands (manually or by keyboard shortcut).
Con No on-the-fly split screen window settings
Unlike some other tiling window managers, SizeUp has no way to change the size of split screen actions on-the-fly. Any time a user wants to change how much room on the screen a window takes up when using the split screen function, they will need to manually change the settings, which can be time consuming.
Con Frequent crashes in MacOS Sierra
aText crashes frequently, and the developer is unresponsive. Not clear if app is supported or under current development: last update was in 11/15; last post to aText Facebook page 10/15.
Con Doesn't work in Mojave
The current version, with no issues. In fact, I've never had any issue with it with Mojave, so I assume when Mojave was released, aText was already up to date for it.
Con Long time no update
Seems to be abandoned.
