While I'd like to believe that the longer barrel is always better for precision shooting, my observations and experience is to the contrary: variations in barrel quality, including chambering specs and execution and rifling method and execution) seem to dwarf the difference inherent in the extra length. Likewise, while a longer sight radius always makes a difference when shooting iron sights, today precision shooting almost always involves shooting through glass.
Frankly, if I were building the rifle you're describing, I'd go elsewhere than a Remington 700, but that isn't what was asked. A good 20" barrel will outshoot a lower quality 26" barrel.