Flat only goes up to a certain baud rate, flat uses vampire clips which are prone to failure (experienced this many times, mounting the cable to where it does not get stepped on is critical for this to work, electricians had a problem if the temp was low in the building and they were terminating the vampire clip, pliable cable at room 70degreeF temp) Since the baud rate is low on flat you can only use it in low rate or low speed applications and the amount of nodes. Also you need to look at the length versus segmented power supplies.
I have used the thicknet round and you do not have a lot of the constraints as shown with flat. Remember there are differences in the round thicknet versus thinnet, which are baud rate and power requriments. I would recommend the Thicknet.