As you said in your question, quantum field theory is very important; it takes the ideas of quantum mechanics and applies them to fields, such as the electromagnetic force (in fact, quantum electrodynamics was the beginning of quantum field theory). Quantum field theory has plenty of evidence to support it, and it is still an ongoing work. String theory, though, is very different. In reality, string theory has very little evidence to support it. Right now, it's basically an idea, and what supports it are the dualities embodied in m-theory. Now, for string theory's advantages:
- Some scientists believe its use of the anthropic principle is an advantage (to explain this in a nutshell, the string theory predicts so many universes some scientists believe this explains away the fine-tuning of, for instance, the cosmological constant, though there is more to the anthropic principle and this issue than that).
- It provides a framework for combining particle physics and general relativity.
- Currently, the model of physics we use is called the standard model. While this model is incredibly useful, there are some things it cannot do - for example, it fails to incorporate gravity. Scientists hope string theory might create a path toward combining quantum theory and gravity.
- The Big Bang doesn't fully explain everything about the beginnings of the universe and cosmic inflation is the theory believed to be our best shot at moving forward. Cosmic inflation needs a particle called the inflaton, whose properties can't be derived from cosmic inflation but might be able to be derived from string theory.
However, it must be kept in mind while reading this is that string theory isn't on nearly as strong of a footing as quantum field theory. There are many kinks to be worked out, and there isn't really any evidence for it. I hope this helps!
Here's Wikipedia's article on string theory, which goes more in-depth into some of this and also explains some of string theory's problems and mathematics: