Paper.js vs Snap.svg

It is written in a language nearly identical to JavaScript, but adds helpful operator overloading to allow you to perform coordinate arithmetic.

An almost-unique feature: can test geometry if contains(), isInside() or intersects() one other object. Offers even hit-testing methods!

Objects in Paper make it easy to extend parent objects and run constructor functions without having to worry about JavaScript prototypal inheritance. Because of this it is easy to make compound drawing objects with their own instance variables and behaviors.

Example: each swimming tadpole follows its own behavior

These features make it easier to create objects that can act autonomously with complex behaviors. This makes Paper a good choice for particle effects and game development.

Elements all have the option of being created with a hash of properties which promotes clean, concise, and contextually local coding practice.

Paper uses a frame based approach to rendering the canvas. This makes it conceptually easier to create objects with animations and behaviors that interact and perform with other objects.

Along with Paper's approach to creating and managing objects this makes Paper especially good for creating applications with complex behaviors with many elements doing different actions at once, and makes it a good choice for making games.

Dmitry Baranovskiy also wrote Raphael (project now owned by Sencha), this is the updated modern version of that library. Dmitry also is a champion directly affecting the future of SVG standards with W3C

Adobe is backing the development of snap.svg

Supports the newest SVG features like masking, clipping, patterns, full gradients, groups, and more