When comparing Noodl vs Principle, the Slant community recommends Noodl for most people. In the question“What are the best tools for prototyping mobile interactions/animations?” Noodl is ranked 11th while Principle is ranked 13th. The most important reason people chose Noodl is:
You can use REST API to get data to your prototype.
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Pros

Pro Use external data
You can use REST API to get data to your prototype.

Pro Free

Pro Cross-platform
It is a Chrome app and you can test prototypes on any device that supports WebGL or Canvas.

Pro Prototype two-device (master — companion) apps
You can setup communication between two devices.

Pro Components
You can build one interactive component and then reuse it with different input values (like images, txt, width, speed — any value you can imagine).
Pro Comprehensive help & examples
You can find videos for the Principle app course here.
Pro Simple Sketch import
Import Sketch layers easily to animate your designs.
Pro Build iOS transitions & animations easily
All the iOS easing info is built-in.
Cons
Con Mac only
Sadly iOS centric.
