I'm still running the standard 6 bolt rear axles - A lot of people say the 5 bolt axles are stronger but at this stage I haven't broken any (fingers crossed). The original axles had a bit of slop in the CV's so I sourced a good second hand set and gave them a clean out and packed them with fresh grease and new boots.
I also picked up another good set of 6 bolt axles that belong to another Nissan, possibly a Skyline or Z car. These were a bit longer than the S15 axles but the inner splines are the same as the S15 axles so I was able to transplant the good CV's onto the shorter S15 Axles along with fresh grease to make up a spare set of axles just in case.