I only ship by UPS to Canada when the customer requests it. Usually, a customer with a business that has a contract with UPS wants UPS.
I no longer ship to Canada by USPS at all. I can track it to the border, usually takes 1 day. Then, Canada Post and Canadian customs take so long to deliver mail that I get hate letters blaming me that it takes a month or more when it should take 4 or 5 days.
In the USA, USPS is good for packages up to about 10 lbs, or for heavy items that fit in a flat rate box. For large items, UPS is best, and for overnight must get there tomorrow, Fedex is best.
However, I get equal numbers of don't ship UPS or don't ship USPS, so I do not see that one is worse than the other, the rate of loss is so low that I see maybe 1 in 5,000 lost, and most of the losses were by Canada Post.