This is one of the highest-leverage account decisions in Canadian investing. The wrong default can leave you with less flexibility, a weaker deduction, or a lot of money in the right account for the wrong job.
If you want a fast rule of thumb, think of it this way: the TFSA usually wins first when income is lower or flexibility matters more, while the RRSP usually wins first when your current tax bracket is high enough that the deduction changes the decision.