Depends if you prefer computer based on paper based.
For speaking, you speak to a person in IELTS but you speak to a computer in CELPIP. I prefer speaking to a real person. For writing, you can use spell check and see the word count for CELPIP as you are typing it on the computer. You have to write on a paper for IELTS. You get headphones for listening in CELPIP. For IELTS, it depends on the centre.
I personally prefer IELTS as it is more widely accepted as compared to CELPIP.