Which method is better for calculating young's modulus : static or dynamic method

Depends. The static method is the "classic" way of testing a material's Young's modulus, but it is destructive in nature. If you are interested in not destroying the test piece then the dynamic method can be used. For example you might use the dynamic method as part of an incoming inspection program for materials supplied by a vendor. But as pointed out by ASTM you need to take into account effects such as temperature history when using the dynamic method.