How wide a lens and on what format will you be using it? (I'm guessing full-frame since we're in the film section.
) The cutouts that make the petal shape do have a function, but as Ramsey said, they're best when actually made for the specific lens.
But they can potentially work better if a round one would have to be very shallow: essentially, if a round one of the same overall depth wouldn't vignette on you, then a petal hood would only be giving you less coverage where there are cuts in the hood.
If a round hood to fit the lens would be shorter than the depth where there are those cutouts, then the parts that stick out further will afford more shading and protection without risking cutting those corners.
Those aftermarket petal hoods tend to be quite shallow indeed, though, so it may be a round one would be better. (If the front element turns while focusing or zooming, then so will the hood when you screw it on: petal hoods aren't for lenses that do this. )
I really prefer round ones, myself, for a number of reasons, among them being convenience and the fact that I'm pretty sure petal hoods can be a bit more imposing to subjects, notably kids and animals. (I haven't made a scientific study of this, but anecdotally, I think it makes a difference: maybe it looks more like 'jaws' or just puts beings off cause there are these parts jutting out at them,) But I'll generally take the extra protection if a petal hood is designed for the lens.