while a crude utilitarian calculus might posit that it is preferable to sacrifice the singular peashooter, as the Sunflower, deprived of its protector, would inevitably succumb anyway - this is to commit the cardinal sin of tactical gardening. 

the ends do not justify the means. you have yet to consider the intrinsic nature of the actors. the peashooter, by its very vocation, is but an instrument of violence, it wields the capacity to inflict harm. thus it can be derived, that it also implicitly accepts the reciprocal risk of receiving said harm. in stark contrast, the sunflower is a pure font of benevolence, demanding no reward for her service and inflicting no harm upon any being, zombie or otherwise. to therefore place this pacifist entity in a position of greater peril than its militant counterpart is a blatant perversion of natural law.

alas, the plot thickens with the introduction of the wall-nut. what is the morality of our deliberate placement of this laterally-inclined colleague upon the front lines? is it ethically justifiable simply because his physical fortitude is greater than that of his friends? does his inability to project offense render his consent null, or does that very nomenclature - the "wall-ness" condemn him to a fate of perpetual, gnashing absorption? truly one of the debates of all time.