Remember that a smaller dot is harder to find fast. If you consider that a typical front sight is going to be 8-11 MOA, a 6 MOA dot is not too big for adequate precision.
Having had both—the larger dot first—I can say for me the smaller dot is definitely better. My front sight is 0.119" wide and in relation to my 3.25 MoA RMR dot it appears to be less than 6 MoA wide, probably 5. That's wide enough for visibility, accuracy, and disappearing when necessary. Eleven MoA wide would be huge.
It is not hard to see in the window. At distance it is much smaller for more precision, since the size of the dot does not change as applied to the target, but the target gets smaller. The smaller dot allows for some "blooming" when brightness of the dot is high and a 3 will appear to be a 4-5. A 6 will appear to be a 7-8. That's too big.
I am also not a fan of big iron sights, like XS Big Dots, as they obscure too much of the field of view except at very close ranges. They are not good for precision, and while hits count for self defense, well aimed shots that hit vitals are better, especially done with speed. The smaller dot does not detract from this capability, and the larger dot does not enhance it.
While a RDS should be co-witnessed with irons in the lower 25% of the window. Usually this requires elevated irons to reach into that sight plane. Those sights should be able to be ignored while concentrating on the dot. The irons should disappear. So they should not be colored, large, wide or confuse the RDS sight picture. If you have a front iron sight that is over 8 MoA wide, you have a very big front sight blade compared to even a 6 MoA dot. It's just not necessary.
The dot should be sighted in sitting on top of the front iron when properly aligned, but it should be
used centered in the window, thereby giving the greatest visibility of the whole target, vertically and horizontally. The lower vertical and horizontal components are not available using just irons, another plus for the RDS.
Supposed advantages of iron sights do not relate directly to best practice uses of RDS. Try to use both size dots before you buy, but if you have any doubts, go small. You'll thank me later.