I am doing a similar build (6SLR), and am going throught the same debate right now. I currently have a Premier Reticles 5-25 and a USO 3.2-17 as well I have owned many Nightforce and Leupold MK4. What I can tell you is that the glass in both the USO and PR are phenomenal and I would say equally as good. USO has a better selection of reticles to choose from and more options to add to the scope (bubble levels, ARD, turrets, illumination, coating/color...)
While S&B are top notch optics, I will not be using them on this build. From what I read on their website, they only have a 2 year warranty. Please correct me if I am wrong because I can't believe that a $3500 scope wouldn't come with a lifetime warranty. The other fact that turns me off is that you have to send the scope to Europe to be serviced if something does go wrong.
March really look like amazing optics but again ,5 year warranty on a $3500 optic just doesn't cut it for me. I really like the 5-40x magnification though.
And then there were two! USO and Premier reticles. I think the new build will be wearing a USO 5-25x58. I also like their MIL scale MPR reticle, it has finer increments that just .5 MIL.
Good luck to the OP, there are many fine choices out there, I hope you find whars right for you!