The Primary Arms Multi-Reticle has three different reticles; the dot/circle - the dot/crosshair - and the crosshair. Of the three reticles the dot/crosshair pretty much has no real use on a red dot. The crosshair, well, its useful somewhat for those use to a more traditional corsshair image but you probably would not use it that much once you get used to the dot/circle and as far as centering your sights on a target the dot is the same thing just without the lines because your going to put your point of impact at the center of the crosshairs anyway which is where the dot is. Overall though, for target range use and general use like hunting, for the price its a good sight.
The Primary Arms micro dot is smaller and lighter then the Multi-Reticle and the dot is more crisp for viewing then that of the Multi-Reticle which may suit your vision just a little better. Its a good sight too for the price for over all general target range and hunting use. It takes up less space on your rail also.
For the dot size, depends on your use requirement (below rounded up):
1 MOA @ 200 yards = covers 2.094 inches
2 MOA @ 200 yards = covers 4.189 inches
3 MOA @ 200 yards = covers 6.283 inches
1 MOA @ 100 yards = covers 1.047 inches
2 MOA @ 100 yards = covers 2.094 inches
3 MOA @ 100 yards = covers 3.142 inches
1 MOA @ 50 yards = covers 0.524 inches
2 MOA @ 50 yards = covers 1.047 inches
3 MOA @ 50 yards = covers 1.571 inches
1 MOA @ 25 yards = covers 0.262 inches
2 MOA @ 25 yards = covers 0.524 inches
3 MOA @ 25 yards = covers 0.785 inches
For red dots, anything within the dot MOA cover size at the range used is considered as within MOA for the group, so if you end up with a group of, for example, 1 inch at 100 yards with a 2 MOA dot then your still at 2 MOA group for the sight even though the group its self may be 1 MOA. Likewise, a group that is 6.283 inches at 200 yards with a 3 MOA sight is considered in a 3 MOA group even though the group its self may be 6 MOA just about. Its different with a scope and crosshair where you shoot for Point of Impact (POI) at the crosshair center and your group is what the impact point is. With a red dot sight you shoot for POI in the dot MOA and your impact group is considered as what the dot MOA is.
The dot cripsness also figures into it. A 3 MOA dot that is crisper and a more defined dot then a 2 MOA dot with a dull edge and somewhat difused at the edges, even though the group would be smaller mathmatically with the 2 MOA dot, is actually more accurate because its eaisier to center on the target visually because you can more cleary see the target area not covered by the dot at the edges allowing more accurate and quicker placement of the dot for the desired point of impact.
By the way, this is where a holographic sight like the EOTech out does the red dot projected in the sight. This is because of the nature of projecting the dot. The holographic appears to be projected onto the target, where the LED red dot is projected in the sight. The human eye tries to constantly resolve the dot in the red dot LED first. On the other hand, the projected holographic dot for a sight like the EoTech is simply "on the target" at any angle of view and the eye does not try to resolve the dot first but rather the target of which the dot appears to be part of, and thats what you want to do, especially for quick target acquistion or with both eyes open shooting with these types of sights, resolve the target and not the dot. For quick target acquistion resolution it should be target/dot and not dot/target while firing. The red dot projected into the sight forces the dot/target resolution even if you don't realize it, its natural and part of how the human eye system works, and automatic because the eye tends to see the closest object first which in this case is the dot projected into the sight, you could not stop doing it even if you tried or think your not doing it. The dot/target arangement tires the eyes more quickly then the holographic that provides the target/dot type of resolution.
I have a friend that has retinal eye disease (which is what caught my attention for your post), and he loves to shoot. We had a heck of a time picking out scopes and sights for him. I have a little bit of an idea of what your going thru, so scope and sight selection is a little more critical for you then might be for someone else and budget might not be your only consideration.
If your budget is in the Primary Arms ranges for the sights you noted, The Primary Arms Micro Dot with a magnifier might be more suitable for you because the 3 MOA dot is a little more crisper then the 2 MOA dot of the Primary Arms Multi-Reticle. The Primary Arms Deluxe 3X magnifier might suit you as well, its the same as the regular primary arms 3X but the deluxe has adjustments for centering the red dot in the magnifier to offset any paralax errors, normally the adjustments aren't really needed by most but in your case they may be needed.
The Bushnell sight, 3 MOA dot also and is pretty clear, but its larger and heavier by its self then the microdot, but comparable in space occupied and weight to the primary arms micro dot with magnifier. It has the same quality glass as the primary arms micro dot. However, the cost is better then the primary arms microdot and magnifier combo.
I'd stay away from the NCStar.
The Nikon is decent for a budget scope.
Even though it costs a lot more, you may want to consider a sight like the EoTech holographic sights also as it will be a lot less strain on the eyes in your case.
Hope this helps, good luck
