When comparing SizeUp vs nvALT, the Slant community recommends nvALT for most people. In the question“What are the best power user tools for macOS?” nvALT is ranked 34th while SizeUp is ranked 57th. The most important reason people chose nvALT is:
nvALT has a simple text box at the top (similar to that of a web browser) and when you start typing inside it, nvALT immediately displays a drop-down box of all the notes that contain the text you are searching for. The more you type, the more the search is narrowed down.
Specs
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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 Powerful search syntax
nvALT has a simple text box at the top (similar to that of a web browser) and when you start typing inside it, nvALT immediately displays a drop-down box of all the notes that contain the text you are searching for. The more you type, the more the search is narrowed down.
Pro Lightweight
Pro Free

Pro MultiMarkdown 3 support
Has built-in support for MultiMarkdown 3.
Pro Powerful note-taking features
nvALT is not only used for simple text editing but also as a powerful tool for taking notes, TODOs, phone numbers and every other piece of information that can be stored in a plain-text file.
This is because nvALT is not designed only as a tool to write text, but also as a powerful search and organize tool for plain-text files.
Pro Right to Left support
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 Not very powerful as far as text editing goes
nvALT's strength is its search feature. It has some basic text-editing features such as cut, paste, find-and-replace and so on. But it doesn't have any advanced text manipulation features available in other editors.
