Interior: 787B>HDI
Sound: 787B>HDI
Speed: HDI>787B
Handling: HDI>787B
Efficiency: HDI>787B
Challenge: 787B>HDI
Fun: 787B>HDI
Overall, the 787B is a better choice for Le Mans, as unlike the Nurburgring Enduro, the track gets boring quite quickly and keeping interested/awake can be a challenge. The 787B's challenging, unforgiving and fun handling style really helps keep the miles ticking over.
FYI: Both car's will easily demolish the AI in (nearly) anyone's hands.