Cooling and acoustic performance are nearly too close to call. The Noctua has 6 year warranty vs the Be quiet 3 year but the Noctua is $20 USD more. Both are top quality coolers and there is no wrong choice here. I'd have to pick the Be Quiet due to price, color and nearly equal performance to Noctua.
http://www.hardcoreware.net/quiet-shadow-rock-slim-review/
http://pcpartpicker.com/parts/compare/be-quiet-cpu-cooler-bk010,noctua-cpu-cooler-nhu12s/