I'm afraid I don't have a firm enough grounding in the comics to comment on how Venom may or may not have been mishandled in the films. I did read the Dark Phoenix saga though. Still, that's only half the story in this case.
I can say I saw Spider-Man 3 and found enough of the film enjoyable to purchase it on DVD. I can't say the same for X-Men 3, which I still haven't seen and don't intend to purchase based on advisories from numerous sources.
What I've heard is enough to make me think Phoenix ultimately wins for most mishandled.
So, one vote for Phoenix from a somewhat unqualified voter.