I worked for both companies and I preferred Manulife. It is larger than SLF for a reason: it's just better.
The other main reason I would want short SLF is that it DENIED me service which is basically fraud (lawsuits coming, right Arvid Shamiri who can be reached at (416) 213-7450).
The other reason is because the ratio chart of MFC/SLF shows a clear uptrend which means Manulife is outperforming Sunlife. The best part is you're "market hedged" and don't have to worry about grampa Powel fucking aorund with rates...