Comparing a cheaper supplier with longer lead times against a more expensive local one

One supplier offers a lower unit price but needs three weeks' lead time. A local supplier costs more and can replenish us in two days. The cheaper option looks great until demand changes and we either run out or hold too much inventory. I'm considering using the long-lead supplier for predictable baseline volume and the local one for spikes. How do you evaluate the hidden cost of holding stock against a quoted unit-price saving?