It is entirely up to you to decide which Canadian insurance company to go with. As you've mentioned, some policies have per claim deductible and some per policy. Those insurance plans per claim are generally cheaper than those per policy.
Although the deductible options and application method varies, I'd say that the main thing is the price to quality ratio.
Try to use
Super Visa insurance calculator to see what are the options on the private Canadian insurance market are available for your parents.
Usually the deductible ranges from 0 to 10,000 CAD and the higher the deductible the lower the price of the actual policy. But I do not think it is smart to go too high on deductible, because if there is an emergency you are most likely will end up paying the bills yourself.
The price also depends on your parents age and whether or not they have any chronic or past sicknesses that are considered "serious" by private insurance providers in Canada.
Hope it will help! ;D