What is key for you? Money or growth opportunity?

Company A - offers X amount, and it's work environment is such that challenges you to learn and grow.
Company B - can offer you 20% more, but doesn't a lot of interesting things, your average turn around time for tasks is a day, two at best.

Which would you choose?

A, not even a choice.

I would go with A because I'm new to the industry and being push towards growth is exactly what I need. But if they have nothing to teach and for whatever reason I didn't agree with their implementations. Then I might go with B and apply the things I learned at A to encourage a more colorful environment in B; assuming the colorful approach will be for the better.

It depends on where you are in your career and what your financial state is. The extra 20% can make all the difference but in general going to an environment you can learn makes for a better choice long term.

Right this second I need that cash, but if I weren't struggling so bad I'd rather be doing more challenging work.

You might need the cash... but is salary *A enough to live from? With a good budget management you can accomplish everything.
Meanwhile you grow and learn and after 5years or so you can switch. Get an higher salary