Send Strings via SPI both ways using two Arduino UNOs(Master to Slave & Slave to Master)
It's not trivial to have both arduinos communicating "with each other" as SPI is a master-slave bus. That means the slave cannot start a transmission, communication must always be initiated by the master instead. Another thing with SPI is that you can't read the slave without writing to it. SPI works like a shifting register: on every clock cycle both devices transmit the byte in their SPDR to each other. So, to read a long string from the slave the master has to transmit the exact same number of dummy-bytes and simultaniously read the incoming stream. This is great if only a specific number of bytes need to be transmitted (e.g. exchanging variables which always consist of the same number of bytes), but becomes complicate when the number of bytes varies. You will then need to implement some protocol that is able to read a variable amount of bytes from the slave (e.g. you will have to tell the master the number of bytes that have to be read).

I think serial communication like UART is perfect for your purpose as communication is real full-duplex, it's no master-slave-system and the common protocol uses a terminating character to end a data stream which makes it ideal to transmit strings. Also the arduino can run several serial interfaces (you might need another for debug output).