Well, the rads are designed to mount the 120's / 140's (depending on the rad) I guess is one reason. The 9x120mm rad will also accommodate 4x 180mm instead. The 9x140mm rad will accommodate 4x200mm instead. But yeah, the fan you linked will be cheaper, maybe half the price. No clear specs on power use or airflow however.
9x noctua 120mm's are rated for maybe 30w. 9x noctua 140mm, around 60w. Good airflow, tolerable noise levels (even at max rpm), and adequate static pressure as well. All of those being at least as important as the initial purchase price. Did consider some other fans. Delta has PDF's for everything they sell so I checked there. Looking at the delta website I couldn't find fans that seemed appropriate at less than twice the watts. So I'd certainly consider other fans, I just don't have any in mind that are overall better options.
Honestly, I'd like to use thicker fans, but again, the included mounting hardware isn't designed for that, so starting out with more standard stuff. Key words -- "starting out" =D
