403r is better than a Romeo 5 and worth the little extra. I've seen a lot of recent run Romeo 5 and msr that have halos around the dot from poor emitter bedding. 403r is a decent cheap red dot. I prefer the rheostat to soft rubber buttons for power.
Be extremely careful when changing the mount, just a few to many in/lbs and you'll strip the threads out of the housing. Pretty universal with the holosun line.
I've got a new 403r and a sig Romeo 7s. The 7s is a slick unit that should be considered. It doesn't have the Halo issues the cheaper 5 series seems prone to have.