Regarding your barrel choice between the 3 versions of the Stretch-16, accuracy will be the same across all versions, fluted version will be slightly lighter (3 ounces lighter). Other than that it would be your preference of color/coating and if you want melonite coating. The stretch-16 does not require an adjustable gas block and you will receive lots of opinions on whether or not they are necessary, but in my humble opinion the gas system really shines if you can tune down the gas.
Regarding JP silent capture spring, I ran one of those for the past 2 years with my 3-gun rifles. Flawlessly, no malfunctions, but it is heavy and the extra spring pak is unnecessary. This year I changed my buffer system to a JP extra power tuned spring with the TACCOM light weight buffer system, which is MUCH cheaper as a whole system and lighter as well, and I am happy I made the swap. Just something to think about. Otherwise my operating system on my rifles is exactly like yours.