Cirrus, Columbia and Diamond aircraft have seats and airframes that can withstand a 26G impact, to meet FAR part 23 requirements. This is probably more than a human can withstand, but they also improve your chances by fitting seatbelts with built-in airbags (as does Cessna). Older aircraft designs will collapse on much lower impacts, and have far more hard edges to brain yourself on. The latter problem is likely to have a significant affect on the outcome of a ditching.