Do the two carts go (down the slope) at the same speed and distance, or does one or other always win out?
I can imagine this being a lot of fun for two kids if there's a random element, but with one cart looking much heavier than the other, I infer there might be a difference in performance. Putting two identival carts (but different colours, as with your old one) looks fairer.
I used to play with Scalextric cars, and they put the same electric motor in everything, no matter the weight of the body, so the Mini would go faster than the Porsche!