so would plastic trunking,
and as for if garage gets pulled down, well you can only work with what you have, unless you know its getting knocked down in the next 6months i wouldnt worry about it.
ive just done an extension above the garage where all cables in the garage were clipped direct to outside wall and i had to remove and resite them all
you cant double guess whats gonna happen in the future
either way sounds fine by me, plastics cheaper and easier to work with whereas metal conduit will get used to hang all kinds of garden tools and paraphernalia on