When comparing SizeUp vs BetterSnapTool, the Slant community recommends BetterSnapTool for most people. In the question“What is the best window manager for Mac?” BetterSnapTool is ranked 10th while SizeUp is ranked 12th. The most important reason people chose BetterSnapTool is:
With this feature, you can move any window into the desired position, select 'advanced: create a custom snap area (using current window as template)' from the toolbar and also edit settings for the snap area. It is also possible to edit the size, position, and presentation of the drop-zone, the shortcut for the custom snap area, and a list apps for which the snap area should be available (eg: only show this zone for VLC).
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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 Custom Snap Areas
With this feature, you can move any window into the desired position, select 'advanced: create a custom snap area (using current window as template)' from the toolbar and also edit settings for the snap area. It is also possible to edit the size, position, and presentation of the drop-zone, the shortcut for the custom snap area, and a list apps for which the snap area should be available (eg: only show this zone for VLC).
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 Vertical monitor orientation snap area bug
The snap feature currently is not working on monitors with vertical orientation. The snap areas show up, but the window never snaps in place. You have to manually adjust each window on the vertical orientation monitor.
Con Costs
BetterSnapTool is no longer free and now costs $3.
