This question carries the risk of comparing apples and oranges. Having said that, one way to make a comparison is described below. First, I would suggest you download the Free Canadian Mortgage Calculator by Vertex42.
In the example, let's say your purchase price was $100000, your current downpayment was $10000, and you take a mortgage at 7% amortized over 25 years. The two options in your question would be decided by either changing the downpayment to $20000 or putting a $10000 additional payment in cell E34.
By making the additional payment, you save a lot of interest only because the monthly payment remains the same. Thus, more of each monthly payment would be applied to your principal balance after your huge additional payment in the first month. Also, your amortization becomes shorter.
On the other hand, increasing your downpayment decreases your total monthly payment given the same 25 year amortization. The interest you pay is higher in this case.