I am also considering adding an EOTech to my shotgun, not a 590A1, but an 870 Police #4407. The 870's do not come drilled and tapped from the factory so I am considering either the EOTech saddle mount for about $50, or the badger ordnance 870 mount which would cost $150 installed plus about $30 in shipping cost so $180. I could save alot of money with the saddle mount, but then I would not be able to mount one of those 6 shot side saddles that are so popular. So I don't know what I will do yet.
Now which is the better sight. I had an M68 mounted on my M4 when I was in the army, but I think when I decide to order I am going with the EOTech because it seems to be much better technology. Wider field of view, shoot with both eyes open, nite vision capabilities with the 550 series, and AA batteries on the 552 and 512.