Multi node bus options
I would suggest either RS485 or CAN; RS485 has the advantage of almost universal availability (if you've got a UART, you can have RS485; if your UART has automatic RTS control, you have a perfect RS485 solution). You can find cheap, small devices that will go up to several megabaud as well.

CAN is a little more robust, but if your microcontroller doesn't have the peripheral it can be an additional cost you're not willing to add to the project. CAN's main advantage over RS485 (IMO) is that in the case of bus contention, a complete message will still get through; If two devices start talking on an RS485 network, nothing intelligible is received and there is no built-in means of bus management, so you have to take care of this in software.

For your given speed and given that I don't believe the microcontrollers you mentioned have built-in CAN controllers, I'd suggest a token-based RS485 network. Essentially none of the nodes speak until it's their turn to speak, and this is done though the passing of a "token" (a short network message granting the use of the bus) to each of the nodes in turn. It's relatively easy to set up, is far more reliable than CSMA/CD and I think you could have something up and running within a day or so.