1979 C&C 30 Mk I - 2QM15
Am I correct in assuming that if you're tying up to pilings, the cheaper Three-strand nylon dock lines will have a longer lifespan than the pricier Double-braid dock lines?
Three strand is perfectly functional. Best to use on old piles. Braided is easier to coil but it clinks on any defect in a pile (like and hang nail on thermal underwear). You can buy a spool and splice it yourself. I've never seen any data to indicate that braided was stronger or more durable.
In addition, if you are into marlinspike artwork you can do more to recycle an old three strand than a braided line.