Should I accept Paypal's exchange conversion or simply allow my credit card company to do the conversion?

When paying for ebay purchased from outside the country, paypal offers to convert the foreign amount into my country's equivalent using their own exchange rate. You also are given the option of simply paying the foreign amount (so the credit card company will convert this amount to local currency). Which of these 2 options would likely end up with the lowest cost to the purchaser? [The two currencies are US $ and CAD]. Does anyone have any experience on this matter?