Amsteel Lifelines

[Brion Toss]

Hello,
1/4" is standard, and will last many years, and the splices will usually fit through the stanchions, so you can splice both ends in the shop. But we often use 5/16" on boats going South, so UV simply won't be an issue. The larger line is also nicer to lean ó or land ó on.
As Bob noted, Johnson makes great hardware for Spectra lifelines. You can splice directly to this, though we usually ring-hitch on, in case you need to replace the hardware or some such. We put thimbles (usually round sail thimbles) at the lashing ends, for smoother take-up, and lash to a bail or shackle on the stanchion, or occasionally the stanchion itself. We use either 1/8" Spectra or nylon lashing twine.
There is a lot of constructional stretch in Spectra, about a foot in 50', so be generous in spacing. But also bear in mind that the splices will take up some length, 2 or 3 inches each, so be sure to compensate for this, especially in the short runs.
For the gate, we usually make a grommet, as the run is so short that eyesplices will back into each other.

Fair leads,
Brion Toss