In general I would say expert knowledge is more valuable. However, I think of knowledge as a pyramid - how high you can go is determined by how wide/strong your foundation is.

In my experience I've found that trying to develop deep expertise requires a solid understanding of many different fundamentals. For example, when I was doing NLP research, I had to learn about distributed systems in order to diagnose problems when training with multiple GPUs, or about dynamic libraries for fixing CUDA errors.