sign-in partner can be used if you are already using a Canadian online banking, you can use the same login details to signup for CIC website, it is much like websites offer signup with google. Here cic chose more trustworthy partners like, canadian banks
GCKEY is a fresh new signup with a new username
Once you are logged in, the profile, options and privileges are all the same so technically dosent matter what you use
If you dont want to create a new username and passoword then use sign-in partner otherwise GCKEY is what most people do since they do not have Canadian crendential to set as a CIC partner.