It’s the age-old question for those with spare cash:
a) Invest the extra dough in an RSP; or,
b) Pay your mortgage down first and then save for RSPs.
This Winnipeg Free Press story suggests that option A comes out ahead in the long-run for most people. However, note the risks and assumptions that apply. For one thing, as the WFP puts it somewhat obviously:
"This strategy only works so long as the interest rate of the mortgage is less than the rate of return on the investment."