Peer to peer means collision detection, master slave means collision avoidance. Collision avoidance in a master slave system is relatively easy and does not require much hard- and/or software. Collision detection require hardware drivers except in the case of soft I2C. Soft I2C is slow and eat up your uCs resources. 485 is not very adaptive to several nodes in transmission mode at the same time (it might work in Antarctica). I2C has a nifty scheme for this, I there are '51s available with I2C drivers, as well as external chips. Another possibility would be CAN there are mny microcontrollers with built in CAN drivers.

I would try to find a way to do master/slave (a pseudo peer to peer could be achieved by frequent polling) it leaves more hair on your head. If it absolutotally has to be peer to peer look into CAN and IIC.

Have fun,
Erik