I found/find fast.ai to be incredibly useful for its practicality, good results, and top-down approach, however, it is sometimes hard to reproduce the results as well as clearly distilling what it is you can actually learn from each lesson. Writing this post blew my socks off as to what was taught in the video, yet it took me quite some time to get it all. So I hope the posts help people with that aspect of the course.

At the moment, I'm learning fast.ai/PyTorch in parallel with Keras/Tensorflow, so at this point, I have no definitive answer to your question which one is preferable. It will probably depend and they will most likely have their own benefits (I know that the boring answer, but I need to get more experience to give you a better answer).

As an exercise I'm trying to write the fast.ai notebooks in Keras, to see how they stack up. Might need to do a post on that as well.

I hope to answer your question better in the future. Could you tell me more about what you want to achieve, I might be of more assistance?