I had a Docter Optics MRD with a 7 moa dot, and its really not as big as it would seem.
I would maybe get the 8moa retical, which would be good up to 50 yards at LEAST (even 100 but the pistol and holding it steady becomes the problem there)
at 50 yards its only a 4" dot, and its still pretty big. The nice thing about the Cmore sights is the replaceable dot modules though, so you can change them for different tasks (if you wish)
A lot of it is personal preference though, you're not really going to have much of a noticable difference in picking up the dot, so a smaller dot might suit you better. The bigger dot will be easier to pick up in various conditions and brighter conditions, although I've never seen a C-more outside before, so I dont know how bright they are.
ETA
They are a little more flimsy compared to Aimpoints or EOTechs though, if its for a range gun/plinker (the AR15) then it will be good, but if you're doing some high speed competition shooting, when transitioning to a pistol, it may bump the lens and crack the plastic arms that hold it...
For the pistol, I might ALSO look into a Docter Optics mini-red-dot sight. They can be mounted directly to the slide of a handgun, and are designed to take the recoil of the slide moving. They are a little bit more expencive though, about 500 bucks with a mount IIRC.... or about 400 bucks with no mount, if its going right onto a pistol, but it needs to be relieved (called melt-in) and then drilled and tapped, so you need to equate that in as well... Unless your slide has a flat top, like an HK USP, although the battery compartment may be open on the sides a bit.
You COULD try a Burris speed-dot - I think they are called - from TSE. They are under 300 bucks, although I don't have any personal experience with them. TSE states in their nomenclature they have proven themselves on high-power semi pistols, but I cant say for sure the quality...
There are mounts that allow you to put the Docter sight right into a rear-sight dovetail as well..