When comparing Briefs vs Keynote, the Slant community recommends Keynote for most people. In the question“What are the best tools for prototyping mobile interactions/animations?” Keynote is ranked 7th while Briefs is ranked 21st. The most important reason people chose Keynote is:
Can import all kinds of assets (images, videos, PDFs, sounds) into mockup without hassle.
Specs
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Pros
Pro Easy to share with clients
Briefs makes it easy to share prototypes with clients through the use of their iOS app, BriefsCase.
Pro Imports nearly every type of asset
Can import all kinds of assets (images, videos, PDFs, sounds) into mockup without hassle.
Pro Animations and transitions
Keynote currently is one of the very few tools that supports animation of elements and transition of views without programming.
Pro Text Styles makes it easy to create typography systems
Pro Smart Snaps
Keynote has alignment grids (aka snap to grid feature) that snaps to end, midpoints, baselines etc of objects and grids. They are super useful when you want to follow a structured layout for your wireframe.
Cons
Con Steep learning curve
There is very little learning material available online for Briefs. The community of active user is small, making it difficult to get any questions you may have answered.