Use the MDF ones - to seal the pine properly it'll need a couple of
coats of undercoat, and may still warp.
I used 'grabs like nails' to put up picture rail in our 30's semi
(obviously at a different height to you!). We also have some curved
external corners - the orginial finish on the picture rail was to joint
it square & fill the wall out to meet the rail.

Phil.