I found that Mortgage brokers mostly can't get the lowest rate because they have to earn commission.
They will offer you comparable rates with small lenders, but those lenders have less advantages. For example, for BMO, you can have the 20/20 option to pay off your mortgage earlier or save $ on penality if you need to break your mortgage. BTW, BMO no longer offers Air Miles for mortgage.
National Bank, Scotia, BMO all seem to be aggressive in getting the mortgage business, so you can very likely get lower rate from them without sacrificing the benefits from big banks. You can usually get -1.5 to -1.75 discount from their posted rate. If they can't get the lowest rate, they will offer free appraisal, cash back etc.. to offset.
For the last 4 times we shopped for mortgage, the mortgage brokers had all failed to meet/beat the rate from the major banks that we got.
If you have a lot of savings in your bank account earning just 4% interest, you might consider something like the Manulife One
http://www.manulifeone.com/canada/mBank.nsf/Public/mone
BTW, don't try to get too many pre-approvals because that could hurt your credit record.