I am getting Isspro gauges, I was wondering which one is more important rail pressure or tranny temp. The truck is only going to get a Smarty programmer for power. I am getting EGT and boost gauges also just didn't no which one is more important for the last gauge for the pillar

 

I would do a regular fuel pressure gauge before a rail pressure. Knowing that would be better than rail pressure if you only had one.
Skip the trans temp unless you are towing heavy or in a hot climate.

 

For the 3rd gen trucks a oil pressure gauge is a need also