IELTS and CELPIP are equally important to be used for immigration purposes, and choosing the either IELTS or CELPIP may be a thing of personal preference.
IELTS is a good choice for those test-takers who prefer a paper-based exam and face-to-face speaking components. And it can be used outside Canada also.
However, CELPIP is for those test-takers who prefer a computer-based exam and are looking for a less stressful test experience.