I have been using one of the Steiners and an Acro P2 side by side for a few months now, and I feel like (for me at least) the Steiner is the preferred optic.
I feel like the Steiner has at least perceptually clearer (or brighter) glass. I feel that the larger window definitely makes a difference here in the amount of light that gets back to me. The other factor that affects this is that the Steiner housing is not as thick from the user’s standpoint. The Acro housing intrudes much more into my field of view, and seems to darken it a bit. This also leads to my next point.
With the housing thicker, I feel I am more likely to get sucked into the optic and begin using it as a frame rather than just using the dot. I catch this happening to me much more often with the Acro than the Steiner.
Perhaps a by product of the brighter view, I feel the Steiner has a (very slightly) crisper dot and it is less impacted by my astigmatism.
I also find the Steiner prints slightly less on my G5 G19 than the Acro. I think this may be in part due to the thicker housing and the fact that I think the Acro sits just a tiny bit taller.
While I feel both are solid optics, I admit the Acro does seem a bit more rugged in construction than the Steiner. While I am perfectly comfortable carrying both on duty, the Acro feels more robust. I was a little concerned that for AIWB carry the adjustment buttons on the Steiner would not wear as well as the Acro’s, which are more recessed, but so far I have not noticed any issues.
I am going to continue testing, but I think for now the Steiner is going to be my preference. I will keep working with the Acro though and see how both hold up over the summer and a hopefully pretty robust firing schedule.