Interesting question, Chris. I think that practicality rules, in most cases. It's easier to plane a conical taper, and check with a straight edge, on a grown or solid built mast. A six- or eight-staved mast will be easier to form if the staves are barrel shaped. A one-piece spun-tapered alloy mast is usually cylinder plus cone shaped, as is a two piece alloy mast, or a composite alloy tube/wooden topmast. A fibre/resin mast laid up on a removable mandrel will naturally have to be conical.
Which is best? My hunch is that a barrel-form, with easy curves, puts the right amount of material in the right places to give agreeable bend characteristics, neither bodily swaying from a point near the deck, nor flicking at the top like a fishing rod. A cone plus cylinder plus cone shape would be very close to this, and for an alloy mast, a cylinder plus cone is very acceptable.