Repetitive strain injury risk, seated vs standing on pedals
This depends entirely on the conditions one is riding in.  Crashing on ice, one is probably better seated as the fall has a lower drop and sliding is easy enough.  In a rock garden, you may be better standing so that you can clear obstacles and not have your bike as an extra factor involved with the crash.  On a busy sidewalk, being seated and stopping more quickly might avoid entangling pedestrians/additional cyclists, etc.  On a roadside, it might be preferable to stand and roll away from the road to avoid the dangers of laying in a heap on a roadside with your bike.  

Additionally personal skill comes into play, people who don't know how to roll with a fall are perhaps more likely to be injured from a higher fall (standing).  However, there are skills that help with a sitting fall as well.  Taking a sitting fall on your side (rolling up from hip to shoulder without slamming knees or elbows) is preferable to going over the bars while sitting or putting an arm out and breaking it.

I think conditions and personal skill with falls will make a larger difference than what position you start in.