It took me too long to notice this thread.
I've always been torn about the "real" nature of the Romulan exodus. Spock mentions that he thinks that Sargon's people were in some way related to the Vulcans, which almost makes me think, in the grand scheme, that Vulcan and Romulus were both colonies of another, more ancient civilization that collapsed (which may or may not have been the same thing as the Preservers.)
It would almost make as much sense as anything if the two worlds simply developed in parallel, on different paths. Left to their own devices, the Vulcans, by nature of their assertion of logical principles, had an easier time of it and became more advanced more quickly. The Romulans, on the other hand, found themselves in a more rough situation and never had a Surak to lead them to logic, causing them to face more infighting along their path to becoming an interstellar power.