1" vs. 30mm - 1" sights were simply first, and then 30mm came along. The larger tube gives a larger field of view, which helps with rapid target acquisition (necessary for certain types of competition.) Either is fine for Bullseye or plinking.
Dot size - An adjustable dot size is more important for Bullseye shooting. For example, I set my dot to barely cover the black of the target. The larger dot is more defined and is easier to center on the black field, giving me better shot placement. However, for most other applications a larger dot simply obscures the target, especially if you’re hunting varmints or shooting long distances.
Parallax – All dot sights have parallax error. It is very easy to see. While on a pistol/rifle rest, center the dot in the sight and sight a target. Now simply move your head to take the dot to the edge of the sight. You will see the dot come off target. That’s parallax. Millets supposedly have the least parallax.
Sights – I believe that Millets are the best, while UltraDots are by far the best for the money. I have an UltraDot and an UltraDot 4Dot. Both are great sights. You can go wrong with a Millet, by spending more than you need to for your application. You can go wrong with a BSA, by spending money on something that doesn’t last or who’s parallax is causing issues. But you simply cannot go wrong with an UltraDot.
Go to
www.Bullseyepistol.com and look at category Tools of the Trade. There’s a good article titled Facts and Figures About Red Dot Sights. Unfortunately, you can't link to Bullseyepistol articles anymore...it just takes you to the homepage. OTherwise I would have posted a link.