Difference between "Shitsurei shimasu" and "Sumimasen"

I would like to understand the difference between "Shitsurei Shimasu" and "sumimasen". Also, please explain which to use when. From this course I understand that "Shitsurei shimasu" is also "excuse me(when leaving earlier)" and it is more polite. But as per my understanding "Osakini" is used while departing earlier. I will appreciate if somebody can explain the meaning of "shitsurei shimasu" also.

They are both used as some kind of "excuse me", but shitsurei shimasu is a little more formal than sumimasen. Sumimasen can be used in nearly any situation, while shitsurei shimasu is reserved for more formal shituations (and even between strangers)

Thanks for the reply, could please also throw some light on why in this course has "shitsurei shimasu" been mentioned as "Excuse me(When leaving earlier)". As per my understanding, it has got little to do with an excuse while leaving earlier and could be used as a formal excuse me but I am trying to understand why is it mentioned in the manner as above in this course?

To complete the above, usually even formally, I have seen them using "Osakini", which is kind of Excuse me when leaving earlier than others.

To shed some light on this whole leaving thing... When leaving before your colleagues, the full expression used is usually: "Osaki ni shitsurei shimasu" as in "please excuse me for leaving early/before you". Now, to shorten the sentence (as the Japanses usually do), they say only either "osakini" or "shitsurei shimasu". It is important to know the entire expression. The abbreviation thing comes along with experience.

Thanks a lot for your detailed explanation, I understand.

I also forgot to mention. Shitsurei Shimasu is also used when leaving, but not in the context of "osakini". In those cases, it's seen as some sort of: "Excuse me, but I have to go..."