Tell me your thoughts on this, please.
Are apples better than oranges? Should designers take baths?
My personal opinion is that Principle is a little easier to get started on and making cool things with and the updates come out very regularly.
Flinto has some very nice features too (that Principle doesn't have) and they were the first with Sketch import.
I ended up buying both but I use Principle more so to me, it's better.
They're different ;) Give the free trials a shot and let us know what you think! Here's a good place to start learning Flinto, you can follow along with source files - https://www.flinto.com/tutorial_videos/14
Flinto's interface is more refined, but not as intuitive (subjective opinion). Principle's UI is pretty rough (i.e. selected artboard focus clashes with asset edge snapping), but very easy to pickup and get started quickly
Whichever tool that can help you deliver what you want and what you need is the better tool.
Personally, I use Principle a lot, and I have to say there is no other equivalent interaction design tool that can achieve close-to-real prototype without writing a line of codes. Surely it can't do everything, but I think it probably covers 80-90% of the possible interactions and transition effect which is sufficient for communication between designer and developers/stakeholder/clients etc.