epoxy, for adhering parts, no question....rod bond!
high build - thick finish
low build - thin finish
High build can do everything a low build can, but a low build cannot do everything a light build can. I use Billy's mix! It's nothing more than a mixture of 1cc, each part of threadmaster high build and pro-kote, so a total of 4cc of finish.
Do yourself a favor, whatever finish you do use, make sure you get the self sealing bottle tops.
Color preserver.....tough subject, you can use it all the time, or some of the time. Best thing to do is a test wrap and put water on it to simulate what the threads will look like when finished. I always use it on my decorative butt wraps, but never on my guide wraps. Over time, the bond between epoxy and color preserved threads weaken and you get a separation of the thread and finish and it looks cloudy.