Some things to consider with optics based on the probable use of the firearm, and you...
1. How close to 45 are you, or do you already wear prescription or reading glasses?
2. Is a HUD red dot that gives you a dot floating on a small pane of glass enough? While small and light, they are not as rugged to getting hit, dropped, etc.
3. What do you do when the battery goes out, do you even want a battery?
4. How far do you plan to shoot? 200 and in, pretty much anything will be okay. 200 and out, glass quality and parallax become an issue. The cheap 1-6s have terrible edge effects and a lot of parallax, so you have to have perfect and repeatable cheek weld...not always possible on a rifle like the RDB.
For anything past about 30 yards, I prefer a red dot that has a focus, or diopter adjustment. 20 years ago, I did not care about that. But being able to adjust a tube red dot for my eyes is a huge benefit to me now. With a straight red dot, I group about 3" for 5 rounds at 100 yards, but with a tube dot adjusted for my eyes, I can cut that down to about 1.5". Same rifle, magnified to 3 or 4x, 1" groups are the norm, which for 55 FMJ is about it. When I get past 100 yards, then magnification becomes more desireable for me unless the targets are big and speed is more important than accuracy.
I like an etched reticle so that if the battery dies, I still have an aiming point. Not a fan of backup irons at all. On a rifle that "must work" I will have an etched/illuminated reticle, a light and a laser, so there is redundancy there. On a plinker, I can take a spare battery to the range. I shot 5 different red dots (or 1x sights) at the range yesterday and I must say that the Holosun is growing on me. If I could take the best features of the Burris AR1X (diopter adjustment and etched reticle) and the Holosun (solar cells and lightweight) that would be a winner.
Since I plan to shoot the RDB past 100 yards, still leaning towards the 1-4 variable power optic and one without large parallax errors.