very good question Jaden. If you’ll look at the Images of both the projects on their first page, you’ll notice that in case of AT89C51, the RFID reader is mounted on a PCB which itself contains a MAX232 and converts TTL to RS232 logic. While in case of PIC18, the RFID reader is directly connected to the microcontroller on the breadboard and gives TTL logic straight away.
In case of AT89C51:
[RFID Reader (TTL) –> MAX232(RS232)] –> MAX232(TTL) –> Microcontroller
In case of PIC18:
[RFID Reader(TTL)] –> Microcontroller
The Square bracket represents RFID Module in both the cases. So you can use either of the configuration in any of the case, only thing you need to take care is that you provide TTL logic to the controller.