The 6 speed gives you the grade brake option and a second overdrive. The second overdrive is not much help in the mountains. ALso when they went to the 6 speed, they changed the rear axle ratio.
The five speed transmission control module can be reprogrammed to give you the grade brake feature.
The 5 speed has a sensor for the transmission fluid temperature that can be read with a ScanGauge. The 6 speed does not have the capability to read the transmission fluid temperature with the ScanGauge.
Pros and cons for each but I would stick with your 5 speed rather than go through the expense to change everything.
