I’ve done both. Leveled this warrior from P1 and had raid gear every leveling phase after. Leveled and raided as Arms until Zirene announced they were burying the spec mid P5.
Made a second warrior in P5 when all the runes were unlocked at level 1 and flew through levels without much hassle. Also leveled as Arms. Didn’t even funnel it much gear outside of some greens. You get a massive power bump at 50 when you pick up the Emerald Warden gear assuming you’ve got the rep for even half of the set.