Which cat will move down?

A kitten is inside each of two boxes, initially held at rest, as shown in the diagram. Kitty together with its box weighs $\SI[per-mode=symbol]{1}{\kilo\gram},$ and Petty together with its box weighs $\SI[per-mode=symbol]{1.5}{\kilo\gram}$.

When the boxes are released, which kitten will move down?

Notes

• Assume that the pulleys and strings are both massless and frictionless.

• The brown line at the bottom of the diagram shows that the other end of Kitty's rope is attached to the ground.

• When the boxes are released, they are simply let go. No ropes are cut!