Three red dice out the front of Salvation does:

4 damage on average using Slaved Turrets or Spinal Armament.

4.703 damage using TRCs assuming you spend the evade as often as you can (i.e. even if you roll 2xdouble hits and an accuracy)

I really like Spinal Armament if using advanced gunnery or on the MC80s & MC80b that can have gunnery teams. Otherwise TRCs are preferable.

I think one consideration that also favours the TRC is standard deviation.

Yup. All valid points. You won't always need to use the TRC's but when you do they are handy. Remember Salvation turns the red die into 3 sides with 2 damage, 2 sides for 1 damage, 1 side with an Accuracy and 2 blanks. Unlike the CR90, the front arc of the Neb won't always need to push extra damage. Simply getting a hit, crit, crit out the front or even a hit, crit, accuracy is good for you.

Forcing the brace or the use of ECM's can be an important part of Salvation's job.