I hear that the salaries in US are generally higher with lower taxes compared to Canada. Is this true for postdoc salaries or other academic positions? Sites such as glassdoor give roughly the same salary for postdoc which is 50000 CAD/USD. What a bout taxes, is the tax really lower in US? For an annual salary of 50000 CAD a person pay ~7600 CAD in federal taxes and for 50000 USD a person pays ~5800 USD. This does not include the state taxes. I am a second year postdoc in molecular biology with a background in physics.
This isn't really an answer, but rather a heads up about how a single average number won't help you decide about taking an offer in one place rather than the other.
First, and foremost, however, there are things more important about a career than your salary. If you make a lot of money but hate the job, then your life won't be very satisfying.
Next, the economies of the two countries are similar, but not identical, and the similarity and ties between the economies keeps personal income (overall) on a rough parity. Otherwise there would be a lot of pressure for academics to emigrate from one country to the other and that doesn't really happen. So salaries may be nominally higher, but so are prices of nearly everything.
Next, as the comments already made make clear, the variability between cities in the two places is probably more important than the variability between the two countries. If you have to live on the US average salary in, say, San Francisco, you will be starving, but in Lansing, MI you will be pretty well off. And whether SF or Lansing is better or worse for you depends a lot on what you like to do with your free time. Likewise Toronto.
While taxes may be nominally lower in the US, the higher taxes paid by Canadians (like Europeans) buys a lot of services that don't exist in the US. The big one is health care which is covered by taxes in some countries other than the US. And the US has been postponing infrastructure investment for so long that our roads/bridges are terrible. I don't know if that is the same in "The Great White North" though. Likewise the US spends far too little on elementary and secondary education. Low taxes in the US is why we can't have nice things here unless you are (very) rich.
Finally, the postdoc is only a way station, not a career. The salary will be "sufficient" even if not extravagant in most places. Certainly much more than what you earn as a TA. But the goal is to move through a postdoc to obtain a more permanent position. With that in mind, the salary is probably the last thing you should weigh in choosing where to postdoc. You want to go where your opportunities for moving to the next step are best.
Think about the life you lead, both the work life and everything else.
Another major difference between job availability. In Canada always difficult to find job both academy or industry. Some point is better in Canada but at the end of the day you need job?. Therefore both countries have mixed benefits. If you want to go academic career then USA is much better choice than Canada. Canada have less resources and weather is another major issue in Canada almost 7 months winter and 3 months summer and 2 months mixed weather. So you can think lot of positive and negative points in both countries.