Closed red or reflex sights are, as the name suggests, closed or enclosed. They do not let in external factors like water, grease, or dirt as easily. A closed red dot is often designed for more demanding use. Closed red dots are typically somewhat more expensive.
Open reflex sights are smaller, lighter, and cover less of the shooter's field of view. However, in poor conditions, they may allow extra elements like dirt, water, or grease to enter more easily. Open red dots are often more affordable and designed for competition use.
For example, all Aimpoint red dots are closed. In addition to closed ones, Holosun also offers open reflex sights.
In short; if you expect and prepare for a situation where the crap really hits the fan -> buy a closed reflex sight.
If you want a more affordable, lighter, and smaller sight or one designed for competition use -> buy an open reflex sight.
However, usage is mostly in normal situations with good conditions, so both will certainly suffice for a long time! Both are suitable for sport shooting and hunting, depending on personal preferences and needs.
