Sine law, when possible, is usually preferred over cosine law due due to its simplicity and ease of computation. The terms obtained are more cleanly separated and also linear in the side lengths.
The main heuristic is that if the angles are the focus of the problem, then the Law of Sines is the way to go. If multiple sides are unknown while one angle is known, then the Law of Cosines can be useful.
More specifically, you will commonly face one of the following 2 scenarios. There are 4 values to consider, 3 being given and 1 being unknown:
- 3 angles and 1 side: use cosine law.
- 2 angles and 2 sides: use sine law.
In cases where you need to relate all angles and sides, you of course use the law of sines since it has the best form for that.
You can see somes examples in Sequences in Triangles.
Also, these laws are very useful in combination with Ptolemy’s theorem in cyclic quadrilaterals. But that deserves its own post.