First question you really need to consider is how you want your rib roast to look? Do you want a consistent edge to edge color ? If so, go with the lower chamber temp (260 or less). If you want more of a bullseye effect where you have an outer band that's well done, an inner band at mid well, yada, yada, yada, center and whatever temp you choose, then go with a higher heat.
At 225ish, figure around 3.5 to 4.5 hours. Though I have to say that those numbers come from using an indirect smoker. Thinking about it more, I'm not sure if the time would be affected due to the radiant heat (even though it too is indirect).