When [math]b[/math] is the ball and [math]c[/math] is the rigid car -
[math]m_bv_b+m_cv_c=m_bv_b'+m_cv_c'[/math]
=> [math]m_bv_b=-.3m_bv_b+m_cv_c'[/math]
=> [math]v_c' = \frac{1.3m_bv_b}{m_c}[/math]
As opposed to the squishy car -
[math]m_bv_b+m_cv_c=m_bv_b'+m_cv_c'[/math]
=> [math]m_bv_b=v'(m_b+m_c)[/math]
=> [math]v' = \frac{m_bv_b}{m_b+m_c}[/math]
If this is correct, then the the rigid car will have a larger velocity. It will not be exactly the number because of air resistence, heat, etc. but it should still be larger than the rubber/absorbant car.