At Canadian schools, we often have Americans applying to our graduate programs. If you don't already know, the way Universities work in Canada is typically:
- Apply for Masters degree programs* during the final year of undergraduate
- Finish undergraduate, start a Masters program (2 years long).
- Apply for PhD programs during 2nd year of Masters**. Note: Even if you want to do your PhD with the same supervisor as your Masters, you have to apply again as it counts as a new program and you count as a new student. So you submit transcripts, get your letters, write your statements etc. again.
- Finish Masters (defend thesis) and then start PhD program, either at the same school or elsewhere.
- Start your PhD program (3-4 years).
* Note #1: A few Canadian programs are modelled after the US style and have direct-entry PhDs.
** Note #2: A few Canadian programs have a "conversion" or "challenge" policy that allows Masters students at the end of their 1st year to declare their intention to remain in the same program for their PhD. If this is approved by the department and advisor (usually course performance, research output etc. is considered) then the student can start a PhD program in their 2nd year of grad school instead of finishing the Masters. This usually means that the student will not receive a Masters at all, and if they leave early, they may not get any degree.
Overall, the total time in grad school is the same, but there's a formal break between the Masters and PhD. Note: The Masters is really like the first two years of a US PhD because it's fully funded and you finish a thesis research project. It's not like the US Masters programs at all.
There's an effective breakpoint in the direct-entry US system too, since most places have quals after 1 or 2 years and comps a year or two later. I personally like the formal break at the Masters level because 2 years of grad school is a good time for the student to decide if they really want to follow this path. It's also a good time for both student and advisor to re-assess their relationship and decide if they want to continue working together or if they should work for someone else or at another place.
One other big difference I noticed in the two systems is that Canadian graduate programs generally spread out their coursework more and start research earlier. A PhD student may take courses up to the end of their 3rd or even 4th year. However, US schools tend to have all courses completed within 2 years, usually meaning research starts later.
Anyways, all of this is to say that you should check out Canadian programs that interest you and apply to their Masters program if they offer both or the PhD program if it's direct entry (I know U Toronto Astronomy works like the US system but don't know much other US-like schools in Canada).
Finally, I don't understand what you mean by whether or not it's worth it if you are not "the best" applicant? Most schools accept many students (5 to 15 or more, depending on the school size) and so it doesn't make sense to say it's not worth it to go if you are not the top one of the cohort. Also, it's not really possible to define a useful metric that would be able to rank all the applicants in order and come up with the "best" one.