I have RMR in 6 and 3 MOA. I first got the 6MOA on a .45ACP thinking I would need the larger size to acquire sight faster. Later got 3MOA for 9mm. Found I can acquire the 3MOA just as fast and it "blooms" less at higher brightness levels. If doing it over I would have only gotten 3MOA.
I've heard someone advise getting the 3MOA as the best of both worlds so you can crank it brightest and simulate the 6MOA with the blooming effect, but it scatters too much light across the entire lens for my liking, not just minimally enlarging the dot.
Another consideration, though perhaps not relevant to your stated range, is whether the dot can be too big for precision. 3MOA represents .3 inches at 10 yards, .6 inches at 20 yards. The 5MOA would be .5 and 1.0 inches at 10 and 20 yards.
So I recommend the 3MOA. If you're in central MA I can join you at the range to compare 6 and 3.