[Ruined]

I'll go with fronts. Center can always be downmixed to fronts and fronts provide the majority of the soundstage for the movie. Its important that you get a decent center channel speaker but as a whole dialogue is generally not as demanding as the music or big sound effects that go through the mains. If you have a mediocre (but properly calibrated for equal volume/db output in relation to the mains) center channel it will be much less noticable than if you have a mediocre pair of main speakers. Similarly, if you have a killer pair of main speakers but only a decent center it will likely be much more impressive than if you had a killer center channel and but only a decent pair of main speakers.

Another way to think of it - if someone was only able to afford one "speaker", would you advise them to get - a monophonic center channel and have music/sfx downmixed into this center channel; or, a pair of stereo mains which dialogue can be downmixed into?