Hi Mike!
I've used Framer and Principle for prototyping UI animations. Both have their strengths and limitations. Framer is the most flexible of the two and I really like how well it integrates with Sketch. That's handy.
The biggest draw back of either of those, or similar tools, is how little can transfer over to production code or assets. You usually have to entirely rebuild it for production, which can be a deal-breaker.
I really like prototyping animation ideas in CSS and JS whenever possible. It can feel slower at first, but there's a better chance some of the code or assets can be reused, or at least be a starting point, in production.