They are both instruments to use to help facilitate procurements.
The supply arrangements, as I mentioned, do some of the steps of a procurement in advance, particularly the pre-qualification of companies. They would be put on a list for a range of services, but it does not constitute a contract.
A standing offer goes a bit further than that and puts in place a pricing mechanism so that goods or services could be procured at a predetermined price and called up by a department. They are different instruments that help facilitate procurement by other government departments.