Play first. If the brilliance of Sheen and Langella has successfully transferred to film, then you won't want to be watching the play thinking "well, this is fine... but they are no Sheen/Langella!", and that wouldn't be fair to whoever they are.
Also the movie will be around forever, but this is your only opportunity to discover the play for the first time. Experience it as it should be, then move onto the celluloid souvenir. :)