In general, I would always recommend getting your spring lines out first. In fact, I'll go so far as to say that you should have at least one spring line on the dock before you run any breast or head/stern lines-always. A spring line can, quite easily, become a breast line, or even end up a stern line. But a head line or a breast line will little serve you when you really need a spring line to work against. The reason I say always is that nature is fickle. If your engine control is going to fail in the ahead position, is it going to fail during the hour trip up river, or is it going to fail in the five seconds before you need to move it to astern to prevent ramming M/V Legal Beagle? There is nothing like a spring line for controlling the vessel alongside. And, as Val says, you should leave them rigged at all times when alongside. In a situation where excessive strain is placed on your lines, ie...collision, large surge, tide/current, the breast lines will always part first. Your springs and head/stern will part last, and prevent you fro doing the severe damage more likely to result from running ahead or astern. The suggestion on polypropylene is a good one as it floats in salt water, but will not take the strain of other synthetics.
“Scientists are people who build the Brooklyn Bridge and then buy it.”
Wm. F. Buckley, Jr.