i think mal is the main one in the tv show, in the cast pics he is mostly in the foreground and episodes like 'out of gas' clearly show that. after all he assembled the crew.
but i can understand how people could think River is more important in the movie, it's probably the marketing's fault (on the cover of the dvd River is in the foreground on the front cover, she's twice on the back cover and she is on the pic on the spine). which is partly true imo, since she does trigger the event in the movie but eventually it is Mal who 'saves the day'. so i guess it would definitely have been about Mal.
as a side note, in the introduction to the movie Whedon mentions the films website and calls it www.cantstopthesignal.com, but now if you type it it redirects you to www.serenitymovie.com. my guess is they wanted to call it that at the beginning but maybe it was too obscure to find on google so they changed to the more straightforward url.