String theory has big problems in that it doesn’t make testable predictions. It might be right, it might be rubbish, we can’t tell. That’s not useful science.
Also, the way that string theory is set up suggests that it will never be a complete theory of gravity (because it doesn’t derive the background curvature of space from the distribution of mass like general relativity does).
Loop Quantum Gravity is trying to be a complete, background-independent theory of gravity and to make testable predictions. If it works (it’s not completed yet), it will be better.
They are all related in some strange way. We have two fantastic theories which explain the mechanics of the very large and the very small… these being general relativity and quantum mechanics. The problem is that they fail when you try and “combine” them… the two theories are completely inconsistent with each other. Quantum mechanics relies on being able to “discretize” and assign a probabilistic theory to particles at a quantum level (very small level). Whereas General Relativity relies on the universe being very “smooth” where space and time are a smooth fabric with no rips or bumps. Because people haven’t found out the best method to “discretize”, or, chop up spacetime into little chunks… we haven’t been able to create a proper *loop quantum gravity* theory. String theory is another way to try and combine quantum mechanics and general relativity by considering 1-dimensional strings instead of points. This appears to be very popular right now but is incredibly hard (working in up to 26 dimensions!!!). It will take someone as brilliant as Einstein to come up with “the theory of everything” which will combine quantum mechanics and general relativity… I’m excited for when that day comes! To answer your question: I believe in certain aspects of each of these theories (given that they are slightly relatable) but not all. I’m not an abstract algebraic physicist so I stay away from quantum gravity! I reckon that, once we understand black hole physics better, we will get closer to understanding how we could relate quantum mechanics and general relativity.