There is practically no difference between the two methods. However, while using, there are some cases where one method will be more helpful than the other.
If you know the direction of the resultant force or vector, and the angles made by the given forces or vectors with the resultant, then you can use the second method. However, you don't have to resolve into the x- and the y- components. You can just resolve each force into their components along the direction of resultant, and add them up to give the magnitude of the resultant force. Be careful about signs. An example of this type of problem may be this:
Two persons on different sides of a river, pull a boat towards themselves by means of a string. The boat moves forward in a straight line. The forces make $\theta$ and $\phi $ with the direction of motion of the boat. Find the resultant force on the boat.
Here, you know the direction of the resultant force. Just resolve the two given forces into their cosine and add them to get the resultant force.
In case where you don't know the direction of resultant, you can use either method. In this case, you'll have to find the x- and the y-components, if you are using the second one. Both shall give the correct answer, and both are accepted.