To become a resident in Canada, you must show your English skills are enough. The Government of Canada documents it in a very clear way. It states what tests and the minimum grade you must have. For English, only two exams are valid: CELPIP and IELTS. I will tell you my experience and based on that you can get a better idea. I don't consider myself very good at English, but I felt more comfortable with CELPIP.
My Experience
IELTS can be taken outside of Canada. This is good, you can do the test before travelling. It evaluates you against British English. However, IELTS is a kind of old-school exam; you use a pencil for the written part and for the oral you are interviewed by a live person. Times are organized per exercise, and they assume you know how to answer them. Don't take this exam without doing at least one simulation, because there are no directions about how to answer. About the exercises, in my exam, there were 4, 2 compositions about your opinion on general matters (for example if it is good or not to travel when studying or to describe how living in Canada has changed you) the other 2 were about to fill some diagrams about a process and missing words while listening to a recording. The oral part is individual, someone will interview you and he or she will record your conversation. In my case, they asked me what I didn't like and how was living in Mexico.
After some weeks you will get your results by postal service.
CELPIP is more of a new-era exam and as far as I know, it can only be taken in Canada (UPDATE: Philipinas and UEA are available now). It evaluates you against Canadian English. It is done on a computer. Timing is closely done by section, this means you can go back and forward answering if the time allows you. The written part has exercises with multiple options and it allows you to use the spelling corrector from the computer, which is good; you only focus on answering correctly. Exercises are more about reading and then answering questions about your understanding or filling in blanks. The oral part is a little complicated because if you talk to a computer you may not feel comfortable. The tricky part about the oral part is to keep speaking (use all the given time); I know this is something difficult for those like me who are very quiet. Oral exercises are about describing paintings, describing things that may happen based on a drawing, complaining by phone about a bad product or gossiping with a friend.
After a week you will get an email with your results, and weeks later a postal mail with your results in a hard copy.
Process
To pay for the IELTS, the same as the exam, is an old-school process. You need to call an application center and you will get a form to fill out. After that, you will need to fax a written authorization to charge your credit card (at least that was the way I did for Algonquin College). You will get later an exam confirmation with a special code you must bring on the examination day.
CELPIP is more modern, you can pay on the web page and get study material there as well. You will get a code as well, don't forget it it is the key to everything.
Conclusion
If you ask me, I think CELPIP is easier to do rather than IELTS.