The TOEFL has been the gold standard of English proficiency exams in US admissions for years, but now the IELTS exam, generally more popular in Europe and Asia, is challenging its supremacy. 

The most significant difference between the two tests is how they test for speaking ability. With the IELTS, the test taker actually speaks with a human being who is trained to evaluate fluency and conversational ability. With TOEFL the test-taker essentially speaks to a computer, the conversation is taped, and then six reviewers grade the session. IELTS maintains that its test is more realistic, and TOEFL claims that because it has only one tester/grader during this section of the exam, the IELTS’  is more prone to manipulation and has inadequate quality control.

Inside Higher Ed has the full story in Taking on TOEFL .