To me, "お腹が空いた" sounds normal and polite as compared with "腹が減った," which sounds informal and sometimes vulgar, depending on the situation.
When you are taking a walk with your friend in downtown, say Asakusa, you may say "腹が減ったな。めしを食おう." But when you are taking a walk with your teacher or senpai, you may say "先生 (先輩)、お腹が空きましたね。何か食べて行きましょう."
We have a saying, "お里が知れる - One's birthplace is revealed," meaning "You can tell what class somebody belongs to from how he / she talks and behaves."
It'd be better not to say "腹へった" to others, unless they're your very close friends.