I have prepared and took the IELTS exam few days ago. I think all who accepts IELTS accepts TOEFL as well, but not all who accepts TOEFL accepts IELTS, this specially in the US where TOEFL is approximately double accepted. It also worth mentioning that the way of testing is totally different so it depends what method is more comfortable with..

overall many say that TOEFL  is harder. personally I like the IELTS way of testing..

IELTS is requied for some countries, also the IBT is required for other countries. If you are allowed to choose one of them, I prefer IELTS because the IBT is more difficult.

I took the IELTS because that was the test accepted by the universities I applied to.

My advice to you is to choose your degree and the universities you want to apply at first, and then look at what tests these universities accept. If you take the IELTS now and then find out you need a different one at one or all of the universities you're actually planning to apply at, you'll have to invest more time and money in the other test.

Keep in mind that you should plan your university applications pretty early anyway, usually people say 6 months to a year before you would actually start your degree. The IELTS is offered on 30-40 dates per year, so you can take it relatively quickly if need be. So 1) pick what you want to study and where, and 2) look at what tests those universities accept, and 3) take that test.