I would imagine that both the .40 and .60 size will take off with ease on a grass strip. The .60 might have a better advantage due to possibly having slightly larger wheels. The sturdiness of the gear itself shouldn't be a factor if they are both trainers, wire landing gear is standard equipment with most trainers. Also, trainers aren't usually overpowered so they will equally perform based on motor/plane size. All this aside, any size wheel can be effective in grass fields, but it all depends on how low or high the grass is cut.