I have the three. Couldn't be easier to locate the dot and plenty of size and contrast. The major advantage for me is it's more precise than the eight. This comes in handy when you're patterning your gun and trying to adjust the point of aim and point of impact. Less margin for error since you're only covering 3" at 100 yds. as opposed to covering 8". That converts to 1 1/2" at 50 yds. and 3/4" at 25 yds.with the 3". The 8" will cover 4" at 50 yds. and 2" at 25 yds.
I'm sure you could get used to either one but I think the advantage goes to the 3" because of precision.
Whichever you get, you'll be getting a solid, quality sight that performs in the field...