As you may or may not know after you receive nomination from SINP you will have to apply to CIC, as it is they who process applications for Permanent residence. The difference between the two application streams is the application process at the CIC-stage. The Express entry stream is entirely electronic and is processed by CIC within 6-months or less through CIC's Web-based Express Entry System, the In Demand Stream does not use the EE system, it is a paper-based application process where you will mail your application to CIO, similar to how things were pre-EE and has a longer processing time.
Moreover, in addition to SINP's 60 points requirement you need at least 67 FSW points to be eligible for EE, so persons that are In-Demand in Saskatchewan, who can't meet the FSW 67 points but may be able to meet the 60 points required for SINP OID may apply. A lot of high qualified persons will have the skills, education and experience but find themselves short of 67 FSW points due to age or adaptability.