The flex ratings might be similar, but the stiffness of the boards is not. The gap narrows a bit as you go longer (because the stiffness of the TRS increases more with extra length) but it will always be considerable.
For your described riding, it depends on whether you prefer agility in the trees (TRS) or stability when bombing steeps (T. Rice). Both good boards, but for different purposes.
I do enjoy a variety of both styles of riding. I should also mention (although might not be applicable) that I train 6 days a week and should have the muscle to spin and turn the board. The guy mentioned with my size either board would be fine due to that. I will be boarding all season at Red Mountain in Rossland, BC.
Your last statement worries me. Will I have trouble weaving through the trees on the pro hp 157? It should still be a huge upgrade and improvement from the 162 TRS reg camber board from 2008 right?