Assuming that table B is exact copy of table A, and that all necessary indexes are created, self-join of table A should be a bit faster than join of B with A simply because data from table A and its indexes can be reused from cache in order to perform self-join (this may also implicitly give more memory for self-join, and more rows will fit into working buffers).
If table B is not the same, then it is impossible to compare.