I think that the movie is better than the series. There are humungous differences between the two versions, though. And, I wouldn't hold them up together as one.
See, Buffy in the movie is a Valley Girl. She is vapid and she hooks up with an outcast/a dude. In the show, Buffy is a little, cool girl who meets up with outcasts--and she is an outcast, herself. Now, the vibes are already different. The series, obviously got into greater detail of the story of Buffy and her Vampire Slaying. But, I think that the movie is short and sweet, funnier and more logical as to the perspective of a valley girl. I guess I associate the name Buffy with Valley Girl.
In the series, I would never peg Buffy as a valley girl--just a girl with a secret. There is a huge disconnect. I love the movie for what it is--a valley girl named Buffy who turns out to be a vampire slayer--Kristy is the best IMO. SMG does not play Buffy as a valley girl as well as Kristy. I think SMG plays a lost girl who is cool, but finds who she really is while making friends with outcasts. I know that it is to be a continuation, like Buffy moved away after the movie, but it still doesn't flow that way to me. Buffy went to the prom in the movie, the only place she would be transferring to, is to college. Anything is possible in programming, but I see two different planes, here. SMG is TOTALLY different as Buffy than KS. I see them as two diff. characters.
As I said before, the movie is more to the point; there is a girl named Buffy who is Valley, and she finds that she has powers leading to danger and adventure, but she is awesome so she wins. The show gets in depth, but there is a little meandering (and the villains, to me, aren't as great).
I think your major point (Buffy as Valley Girl) was one of Whedon's issues. He did not envision her as vapid and vacuous, but rather, a popular girl who's fate would affect her agenda. Fans have two visions of Buffy: One as directed by Fran Kuzui and the other as directed by Joss Whedon, who is the creator. Its obvious the show is truer to the creator's vision than the movie. After that, its just preference. I suspect one of the reasons the show is more popular is that people find SMG's Buffy more realistic. The Valley Girl (as archetype) is known to most people by reputation. Unless you live in SoCal, you're unlikely to have much experience with the personality. I was born and raised in SoCal and I found the movie's version of the MallBrat kind of mean spirited. Buffy's friends were lacking the joie d'vivre of the adolescents they were depicting. Kind of a New Yorker's version of Southern California teen-agers.
BTW, you suggest that in the TV show Buffy is an outcast. If I may refer you to Season 3 Episode 20, "The Prom" I think a different evaluation can be made. Buffy learns that who she is (The Slayer) is no secret. Buffy is not an outcast, she's more like an extreme version of the Big Girl on Campus. This is why Cordelia (the consummate Popular Girl) eventually joins the Scoobies; Buffy is the true power at Sunnydale High School, so much so that she exists in a place her classmates can only observe.