The MAC of the device is not available, nor is any other particularly useful identifier. However, since "these devices do not have the same functions," they should have different services that they advertise. When calling scanForPeripherals(withServices:options:) you should be passing the specific service or services you're interested in. This is much better for performance, and also will automatically filter out devices you are not interested in. Passing nil for serviceUUIDs should only be done for a generic BLE scanner.
If you control the device firmware, you can add services to identify the type of device, or add information in the manufacturer's advertising data to distinguish the devices during scanning.
If these devices advertise the same services and are otherwise identical, then you will need to connect to both and query them to determine which device you wanted. You still will not receive a MAC, however, unless the device provides it via some characteristic.
Typically, a given device will continue to have the same CBPeripheral UUID, and this can be used to reconnect to previously known devices. However, if the device never pairs securely, this UUID is not always stable, either.