I like 1/2" double braid nylon for my spring lines, and 5/8" for the dock lines. I find I can tension the 1/2" springers but can't get enough tension on larger lines to stretch them at all.
I have the WM pre-made lines and I personally hate them, they are too soft, chafe too easily, and pick up splinters off the dock and try to transfer them to my hands when I am handling the lines.
I think buying white double braid off the spool and paying for the splices (can't do it myself, no interest in learning how) is the cheapest and strongest line. I am replacing my lines this year, my springers are 25', and the dock lines are 15' (stern) and 25' (bow) so there is something to toss as I near the dock.
I am sure I would feel differently about the lines if my boat were heavier, but those size right for 30' and 20,000 lbs in my opinion. JMO