Recently I had two interviews for a job, one at a company where I worked before and a new one. At the old company I can get a higher salary (8$/h more), than for the other. But in the new one, I can enhance my experience for machine learning. So I get paid for "learning" and working on, "for me", new stuff, but I get less salary.

I know the old firm very well, and can start right away with coding, but is probably not challenging.

So I don't know what to do...