You can go for either as they are near in performance for the viper an noctua. They have near identical stats if compared spinning at same rpm speeds. The viper is just capable of going to 1900rpm which in turn give noise. The noctua is accurate with wind flow as its focused designed so they are real good with tight spots like rads, filters, grills, heatsink etc etc while being quiet was what I came to know about the noctuas along with their fat 6 year warranty and quality construction.
Yeah I was SO tempted to go with the noctuas but couldn't really justify the £10 more for basically the same specs. I'll try them out and see if they make a difference to my GPU temp if they don't make much difference i'll try out the Noctuas