I can't speak for the Scythe, but Noctua is a fantastic company and the NH-D15 is pretty much the best big air cooler in terms of cooling capacity vs. noise you can get. It also comes with it's own excellent TIM and a six (!) year warranty. Heck, when I changed to a new socket Noctua even sent me a new mounting kit free of charge.
In this comparison, you can see that the Scythe beats the NH-D15 by about ~2C, but the Noctua is significantly quieter:http://www.overclockers.com/scythe-fuma-heatsink-review/