"She sounds like somebody who will retire with a nice sum and needs to take everything into consideration."

Absolutely and the answer is probably a combination of RRSP, TFSA and a cash account.  

Further your anecdote  about the lady losing her GIS actually supports my point.   Low income earners are among the last people who should be using a RRSP account as it is next to impossible to realize any tax savings and it only increases the likelihood of clawbacks.

Meanwhile those on the higher end of the middle class and up stand a reasonable chance of contributing at a higher tax rate than their retirement rate.  With the worst case scenario of breaking even.