I want to send messages back and forth between two STM32s. Probably STM32F407VET6:：http://www.kynix.com/Detail/291138/STM32F407VET6.html, but it would be nice if I can try already something for a simple one (STM32F103C8T6).

My requirements:

round trip delay time: preferably 1 ms max (excluding processing), for messages with a payload of around 8 bytes.
distance around 2 meters
speed: pure data: around 128 kbps (excluded overhead) so probably around 200 kbps
I checked between SPI and I2C, and seems that SPI is easier (to setup) and I have enough GPIO pins.

However, I read that 2 meters and fast data rate might be a problem. Also I cannot find info about the round trip delay.

Of course I can check all this myself, but it saves me a lot of effort to set this up.

For the cabling between the two STMs I am intending to use an RS232 connector (and probably cable), but if there are better alternatives, please let me know (UTP cable maybe and internet connector?). Or are there more appropriate cabling/connectors for this?

(update, link to MCP2551 according answer remark) MCP2551 adapter: adapter