The option for the insurance company to use aftermarket vs. OEM parts will definitely be stated in your policy, no doubt and its not worth arguing, it is clearly their option....most people never read the fine print.
Here is how to get around this issue, the vehicle must be restored to all OEM tolerances or it has to be considered a total loss. The person(s) who can veto the insurance company's aftermarket vs. OEM parts choice is the repair shop, if the aftermarket parts do not meet the exact tolerances/fit of the OEM parts the shop can reject the aftermarket part and require the OEM replacement.
I had this same issue on an bumper replacement for my ML430, the insurance selected aftermarket bumper fit poorly and the shop rejected the part and then used an OEM replacement instead...done and done.
You will have to jump through the aftermarket hoop but talk to your installer of your expectations...they will know exactly what you are talking about....most aftermarket parts never fit properly and repair shops reject them all the time.
Yes, there are OEM specific replacement policies available but one has to ask and pay for such policy.