When comparing Notational Velocity vs TotalTerminal, the Slant community recommends Notational Velocity for most people. In the question“What are the best power user tools for macOS?” Notational Velocity is ranked 11th while TotalTerminal is ranked 20th. The most important reason people chose Notational Velocity is:
The interface is clean and unobtrusive.
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Pros
Pro Simple and minimalistic
The interface is clean and unobtrusive.
Pro Free
Pro Syncs with SimpleNotes or Dropbox
Pro Allows quick access to the terminal system-wide
TotalTerminal is a plugin for Terminal.app. It provides persistent Visor Window which slides down when you press a hot-key.
Pro Quake style window show/hide
You maybe know or don't know Quake from Id Software. But this game had the best idea to hide/show a terminal window. Simply using full width but 26 rows. On show the widow slid down from top of screen. On hide it mode up to top edge of the screen until it disappeared. 2nd next best was the semi translucent background. You could still see through the terminal window while it was shown.
The same style is now available in TotalTerminal and iTerm2 (OSX Terminal replacement - no Terminal plugin).
Cons
Con Feels a bit outdated
The last release was from 2011. Although due to its brilliant simplicity in doing what it's designed for, some might think there hasn't been any need for an update, as long as it still works with the newer OSs.
Con No longer under active development
Con Doesn't work on El Capitan with SIP on
Apple introduced a new security feature in OSX El Capitan, called System Integrity Protection, that, among other things, does not allow augmenting Terminal. While it's possible to turn SIP off (instructions on how to turn off SIP and get TotalTerminal working on El Capitan can be found here) it is not recommended.