A lapbelt might be better for an adult in the sense that our hipbones are more developed and can withstand the forces placed there by the lapbelt better than a child, BUT an adult is worse in a lapbelt in the sense that our upper bodies are larger and heavier and we will fly farther forward, and with greater force, so there will be much more force placed on our backs, and much higher likelihood of our heads coming in contact with something dangerous within the vehicle.
If there were children in the car in carseats, I'd rearrange and place one carseat in the middle. If the kids were booster-age and needed shoulderbelts, I don't know. Tough situation, for sure.