Paint or Stain Exterior Timber

I've recently built a timber retaining wall with treated timber (pine) and I don't know what product to use to protect it from rain etc. and increase it's life. It's already started to warp in a small area, so I want to either stain or paint it and I'm not sure what best approach is?



Would just painting it a dark colour be okay OR would timber stain be better OR wood preservative?



I did stain my deck a couple of years ago and it looks good, but it still has splits along the railings for some reason.



Thanks for your help :)

Paint or Stain Exterior Timber