These are two of the greatest films ever made. Im going with Boogie Nights because i do watch it more often, and because its the sort of film that you recommend to someone and they ask "really? i should watch that?"
Furthermore, I want Boogie Nights to win because there should be a second chance episode sometime (right?) and I want both of these films in. If There Will Be Blood loses, i think itll be more likely to be voted into that Second Chance episode than Boogie Nights if Boogie Nights loses.