I think you're talking about the GC Key vs IRCC portal. IRCC is the newest portal. The difference is IRCC, they have made a large part of the application process which you have to type/fill-in your answers in the website. The GC Key, you have to download the forms (PDF, photo or docx format) and upload it to the particular section eg. you fill out a family info PDF form then upload it to the family info section.
I applied via GC Key a few years ago. However, my passport expired with the visitors visa. I'm planning to apply via the IRCC portal because of some feedback in this forum about having a slower response when they submitted via GC Key. I'm not sure if this is the actual situation but I'm assuming they would prioritize the newer portal.
