either or
While 3 strand certainly is the old school standard, I think personal preference may have some say in the matter. 3 strand does have more stretch and is cheaper, and easier to splice, all valid points. If you are in the Chula Vista marina, or any protected tie up, a lot of people prefer braid, probably just because of hand feel and looks, plus it comes in many more colors. If you're in a really rough tie, like Lake Washington in Seattle (choppy in winter), you should probably stick with 3 strand and even add in some snubbers. The normal minimum would be (2) 15 foot lengths (bow and stern) plus a 25 foot springline. 3/8 inch may be adequate, but I would recommend 1/2 inch. If you travel a lot and tie up in many different situations, you'll always want to be prepared with much more than the minimum, probably X2 at least.