The theme of a SOP is geared more towards the first time visa applicants. In this case, the applicant will more likely talk about how the advance studies is going to complement past academics and how it will brighten up the future career scope. In addition, various other topics will be addressed such as capability to finance studies & living cost, why particular program, why Canada, why particular institution etc...
The theme of a LOE is geared more towards the re-applicants (after their visa has been rejected in past). In this case, the applicant will start first-half of the letter by explaining what went wrong in previous applications and on what ground it was rejected. How those issues are addressed in the present application. In the later part of the letter the usual SOP stuff goes in.