In other words, which one is better depends on the role you're intending for the weapon.
Improved Range Finder is better for, say, an assault rifle where you may need to engage enemies at range, especially while either closing or fleeing from the enemy. Fluid firefights are what you should be thinking here.
Vision Magnification is better for, say, sniper rifles, where you're looking to make accurate shots at long range. Hunting or ambush situations are the key here.