Please keep in mind that those exams do not provide certificates, but instead they issue a TRF (Test Result Form), which isn't quite the same although it's still widely accepted across the globe.

Although IELTS is a British/Australian exam and TOEFL is an American one, they're both accepted internationally. So, there are loads of colleges and institutions in the States that accept IELTS, and loads of Commonwealth ones accepting TOEFL.

Now, some stuff to think about (for the following comments, I'm only speaking about TOEFL IBT, which is quickly replacing the older PBT virtually everywhere):

* IELTS accepts both Commonwealth and American spelling; TOEFL only accepts the latter. If you write with American orthography (e.g. color, sulfur, edema), you'll be comfortable with either one; if you write with Commonwealth orthography (e.g. colour, sulphur, oedema), then TOEFL will be harder as you're gonna have to 'translate' many words.

* During the listening section, IELTS randomly employs a wide range of accents (English, Welsh, Scottish, American, Australian, Irish, New Zealand), some of which are rather informal. TOEFL uses Standard American almost exclusively. So, again, it's up to you.

* The speaking section of TOEFL IBT consists of talking to a mic (they record what you say and then assess it in New Jersey). IELTS consists of an interview. Moreover, TOEFL asks you to talk about chiefly academic matters (e.g. speak one minute about bee reproduction or a psychological bias), while IELTS (both Academic and General) has more quotidian questions (e.g. 'tell us about what you like to give and get as birthday present'). For many people, talking to a mic's better than face-to-face, for others, it's the other way around; same for the topics: many feel 'at home' when speaking about scientific topics but get confused with a more everyday dialogue, or viceversa... so, again it's up to you.

* TOEFL is cheaper, quicker to take (about 3 hours, a little more or a little less), and results get to you in less time. Moreover, it's graded from 0 to 120, which is much more precise than from 0 to 9 (although it's got half-bands). In spite of that, many people still prefer IELTS and feel the extra money spent is worth it.

Of course, there's also the chance of sitting both... that way you'll be way more competitive - many people have aced the IELTS, many people have aced the TOEFL, relatively few have aced both.

http://answers.yahoo.com/question/index?qid=20091014234231AAJHv4p