Go with the fixed Daniel Defense rear sight. It's cheap, lightweight, simple, no elevation adjustment, only has windage. Simplicity is a good thing here, as these are backup sights. Dial elevation in front, set windage with the rear, you're done. Personally I think it's a bit of tactical masturbation when people feel like they have to have some crazy match grade, 600 meter backup sight. A majority of us are not doorkickers and will not likely ever need the backup sights, and if so will never need a "backup sight" for a 600 meter friggin shot (uh how about GTFO of Dodge at that point!). Seriously, less is more in this case.