altawashm,
They are used for different purposes. The SIM slot would allow the car to operate as a phone, where the bluetooth feature allows your car to act as a speakerphone extension of your cellphone.
Personally, I would prefer the bluetooth feature, as this does not require a new line on your plan, or swapping out the SIM on your phone, etc. I use the bluetooth fester regularly on my BMW
From what I can tell, these 2 features are mutually exclusive. If your car came with an inbuilt SIM slot, then there was not a bluetooth feature, and vice versa.