Arenado at least never K'ed at more than 15% at any level. Gallo has K'ed at 33% for his career, which is a virtual death sentence for a hitter. There's just far too much swing and miss for him to ever last against advanced pitching. IMO he'll come crashing back down in AA and fail to hit his body weight in the majors, before eventually becoming a fireballing RP.
Arenado is fantasy relevant, but IMO he'll be a 2nd division 3B over the long-term. Career 78 wRC+ away from Coors so far (96 at home). I don't see him consistently topping 20 HR.