I agree that typically right handed people get the left shoulder so that they can rummage around with the right hand. But if you are reasonably comfortable using your left hand then the right shoulder bag will leave no obstruction when looking over your shoulder at traffic.
The people at seagull are also really good at answering questions so if you have any issues that don't get answered here just ask them. Ortlieb probably has good support too. Though they wouldn't be as helpful for inter company comparisons like we are discussing.