Sine law is if you know one side and its opposite angle. Once you know this ratio, you can work out others if you have another angle or side.
Cosine law is if you know two sides and the angle between them.
Even though the equation is slightly less simple, I prefer using the cosine law when solving triangles since inverse cosine returns an angle between 0 and 180. If you use the sine law, you will have to use the inverse sine and that could cause problems since inverse sine returns a value in the range -90 to 90, e.g. if you're trying to find an angle = 95 degrees. Inverse sine will give you 85 degrees instead of 95 degrees, which could be quite difficult to spot.