I guess it depends on the type of Silver Arrow? I mean, both are serious contenders, but the Silver Arrow SB-E is definitely a different beast.http://www.vortez.net/articles_pages/thermalright_silver_arrow_sb_e_extreme_review,9.htmlhttp://www.xbitlabs.com/articles/coolers/display/thermalright-silver-arrow-sb-e-extreme_5.html#sect1
Note, though, that the SB-E isn't exactly meant to be quiet. The original Silver Arrow is more pleasant to the ears, and is still a give good punches: http://www.xbitlabs.com/articles/coolers/display/thermalright-silver-arrow_5.html
You'll find a lot of reviews that show the Silver Arrow (both types) trade blows with the Noctua NH-D14. I think it's safe to say they are, roughly speaking, equal. I would probably be happy with either product, so I say just get whatever is cheaper. The tinkerer in me, though, says, since you already have a Noctua, go ahead and buy the Silver Arrow and give it a shot. Worst case scenario, you still get to tell us your experience as to which was better.