During the conciliation process, the parties appear before a public interest commission. This third party hears the parties and submits a report at the end of the process. However, this report is non-binding on the parties: They are not required to follow the commission's recommendations.
During arbitration, people also appear before a third party and state their case. However, this third party's report puts an end to the process, meaning that it is binding on the parties and fundamentally becomes the new collective agreement. It is possible that the bargaining agents prefer to have the right to continue to negotiate for strategic reasons.
Once the report is submitted, choosing conciliation can potentially lead to a strike, but it can also lead to further discussion on the aspects covered by the report. It is a strategic issue for bargaining agents. They must ask themselves if it is preferable to give the last word to a third party, or to retain the possibility of continuing talks with the employer, even after the third party has given its opinion on the work conditions and the disputes.
All in all, these are the distinctions to be made.