Super thanks for telling me about the branch<->speed relation.I have read a couple of pages hinting about this but it was very vague.
Give a go with Can Bus... Very simple and cheap to implement. Also very resilient for interference.One good advantage over RS 485 is that it implements not only the physical layer but also the data link layer, offering retry, crc and frame format out-of-the-box.
One disadvantage with CAN seems to be a very short stub tolerance. 0.3 meters to be exact.http://digital.ni.com/public.nsf/allkb/D5DD09186EBBFA128625795A000FC025
I did not understand why you would need 2 CAN Bus on each Arduino node. You only would need data in case you have two CAN networks and wish to router traffic from one to another.
and still remember that RS-485 only implements the physical layer, you going to need to "frame" your messages (http://www.gammon.com.au/forum/?id=11428), where CAN bus does that for you.