The answer is most likely neither, but... physics!
So, a Hummer (H1) weighs around 8113 pounds, or 3680 kg, and can travel at a top speed of 55 m/s. This results in a Hummer having a max momentum of 202,400 kg*m/s.
A Lamborghini Aventador, on the other hand, weighs around 4085 pounds, or 1852 kg, and has a top speed of 97.22 m/s. The Lamborghini's max momentum is 180,051.44 kg*m/s.
So although the Lamborghini can travel at a much higher speed than the Hummer can, the Hummer's weight overpowers the difference in speed. Therefore, if you have a choice for some odd reason, to choose what car to get hit by, go for the Lamborghini. Either way, ouch!