There is no real tangible benefit to dating her aside from the requirement of the story-related missions.
**IMPORTANT NOTE** If you want to get the perfect 100% girlfriend fondness that's needed for the XBOX360 achievement, you need to have Michelle's fondness at 100% before the mission "Snow Storm".
Food (Burgers/Cluck'n Bell/Restaurants and Diners) B. Michelle and Kate are met through the game storyline and while dating them don't provide any real tangible benefit (Hot Coffee not withstanding) they do contribute to a 100% game completion.
[6a] Michelle Date Setup: Part of the game storyline, so its automatic.
The Sports cars are usually the fastest cars around and while they look good, they may not appeal to the women who expect high class cars.