For a background scene on iOS, should I use terrain or just mesh?

Im making the art for an iOS game, its a beach scene in a toon style. Lightmaps only, no real time lights. Some alphas on the trees. Maybe a few bushes but no modelled grass or anything small with alphas. The terrain is just a background, the players position is fixed so they will not walk up to any area of it. Is it more effiecent for iOS to use the terrain tool in unity? Or shall I just make the background as a model with one big 1024 png texture?

Thanks