The tube body is what those 30mm and 40mm measurements are. It depends on what mount you use and what sight picture you want.

Absolute cowitness means the iron sights line up in the center of the tube. Lower 1/3 cowitness means the iron sights are slightly lower in the tube body. When you look over the top of the sights the dot will be higher in the body of the sight, and the benefit of the lower 1/3 is the dot is more centered in the field of the sight.

Reflex sights are much lighter and don't block as much of your field of view, but they tend to wash out in bright sunlight and I don't think they work as well to remove parallax.

Tube style sights are the opposite. They block a bit more of your vision, weigh a little more, but the dot will be more protected from light. The more you pay the more quality you get.

If you want the cheapest one possible your best bet is to spend $30 or so on a Leapers tube red dot model and add a 1/2" riser on top of the rail for cowitness. The styling of this sight is very bland, but it works for the price you pay.

If you want something a bit more cool looking you can get an Aimpoint clone from Primary Arms for $70-100. These look great, work on the 5.56 AR rifles (if you have or get one of those) and are pretty good quality as well. I know these sights are also mostly parallax free (parallax being the dot not being on zero as you change the angle you're looking through the glass of the sight).

I chose the Primary Arms M3 clone on a high mount for lower 1/3 cowitness. I also have the Magpul backup flip sights. These pictures are a little out of focus, and my camera really picked up on the distortion of the glass, it's not that green in reality. This is a 30mm sight (the tightest part of the tube is 30mm in diameter).

Front sight only

Looking over both iron sights

Using the iron sights with the dot off

And how it looks on the rifle (this is the high canted mount)



Hope this helps, good luck with your purchases!