I2C is a lokal communication bus, and used inside a few meters. If the distance is greater you need a more robust bus, or more hardware to split the bus into sections.
RS485 is defined as a 2-wire half duplex multidrop bus, normally master/slave, and you will meet som problems regarding collisions from a multimaster solution. The transmitting unit(s) can not listen for collisions while transmitting, they will only se their own data. To solve this problem you need to implement a protocol based collision detection. There are several approaches for this, and what you select will depend on the speed and contents you want to send.
RS422 is the corresponding point-to-point variant of the RS485, normally a 4-wire solution.