With SIS, you plead guilty to the reduced offense charges — but there is no sentence issued by the court. As a result, the offense gets completely removed from your record as long as you don’t get into any trouble during the probation period.
On the other hand, SES requires you to plead guilty to the reduced offense charge and sentences you to that offence. But it does not require you to start serving that sentence – as long as you keep your nose clean during the probation period.
In other words, if you don’t get in any more trouble during the probation period, with SIS the lesser offense you plead guilty to is completely removed from your record at the end of your probation. But with SES, the lesser charge stays on your record forever.
Both SIS and SES can be very helpful – as long as you stay out of trouble. If you get caught doing something else and get picked up by the police, things will get worse. With SIS, the court will sentence you for the lesser offense you plead guilty to, and it will probably be worse because the court will know you also have another offense. With SES, you will have to start serving your sentence.