I've owned both and both kuiu and firstlite are good. Kuiu's sizing runs small though in the base layers. Firstlite's mediums fit me perfect, not to tight and not to loose and the SLEEVE length is perfect. However, no zip neck!
Kuiu's has the zip neck but a medium fits tight and more importantly and one of my pet peeves, is the sleeve length is to short! Nothing worse, imo, than a long sleeve shirt with sleeves halfway up your forearm. The next run I do intend on trying the larges.
Now everyone seems to like about a 225 or 250 weight for the next layer, but for me it is a total waste to go that light. Not enough warmth and since I move slow a heavier weight is not an issue. So, firstlite's sweater is my next layer, its a 400 weight.
As far as synthetics go, I have found that after 2 days they stink to much. I can hardly stand myself, I don't have this problem with merino wool which seems to stay scent free at least twice as long.