Oh? While I'll admit Gren's own Rocket Upgrades will give him an edge, I think as far as firepower and durability goes they're rather even. Their Semblance is the real game changer, which can be altered a situation in various ways. 

For example, if we're fighting not just in an open arena, but in a place with cover and different levels of elevation, that could give your character tactical advantages. Not that Gren is without either, but his plans tend to be short-term.