In my experience, reflex sights in bright sunlight tend to "wash out" the dot a bit especially if the light comes from behind, the tube type of sight like the Bushnell TRS-25 and Ultradot, don't "wash out" nearly as much.
The Burris FF2 has to be unmounted to change the battery as it is only accessible from the bottom, the Burris FF3 fixed that problem.
IMHO, very little difference between the Venom and the FF3.
For most people that I have been around, the reflex sights tend to have the dot be acquired quicker than the tube types.
I agree that the smaller the dot, the better accuracy potential especially as distance increases.
Out of curiosity, what is your intended use; target, hunting, plinking, competition, etc.?