What is the difference between NumPy object methods and NumPy function calls?
There is a notable difference between numpy.sort and ndarray.sort: the former returns a copy of the array, the latter sorts in-place.
For other methods, one can use equivalent functions just as well. The function form will accept array-like collections that can be converted to a NumPy array; this is sometimes convenient. On the other hand, this comes at the expense of a few more checks and function calls, so the method form should be a tiny bit faster. In practice, this is probably negligible; for me the deciding factor is usually that methods take fewer characters to type.
Some mathematical operations are more naturally written as methods: compare np.transpose(A) and A.T, when A is a 2-dimensional array.