Slay hit it right on the head. I have done testing with different combinations of stifle and response. 3x Stifle and 2x Response are what I currently run in my U/W Landstill build. Yesterday I got the deck into top 7 of "WIn Ed Fear's Lotus" tournament and alot of it was due to both Stifle and Response blindsiding the opponent across the face. I can't say how many times I Stifled fetches and stifled Lacky's ability to get an extra turn. I also smashed Matrons and Ringleaders with stifle. Response worked very well as well. Against wasteland it was very effective, especially when I tricked them into attempting to wasteland one of my lands. In one of my matches I actually secured a game by Responsing STP on my Factory. Overall, 3x Stifle and 2x Teferi's Response is happiness.
:D
Cheers.