Neither wins everywhere. Blown cellulose is the cheaper way to reach R-49 on a vented attic floor once the leaks below it are sealed. Spray foam wins where one thin layer must seal and insulate: roof decks, cathedral ceilings, knee walls, rim joists, and crawl walls.
More detail
This answer is the short version. The full side-by-side, with costs, the paperwork Ohio's code requires for each product, fire rules, and where each one fits in a Greater Cincinnati house, lives on our spray foam vs cellulose comparison. The core trade-off fits in a paragraph. Cellulose is recycled paper fiber treated with borate fire retardants, roughly R-3.5 per inch once it settles, inexpensive to blow deep, and easy to vacuum out later, but air moves through it, so every top plate, light housing, plumbing chase, and attic hatch underneath has to be sealed first or the R-value on the bag chart overstates what the house gets. Spray foam costs more per unit of R-value but is its own air barrier, which is what counts in the places cellulose cannot fill or cannot stay put: the underside of a roof deck over an attic air handler, a shallow cathedral rafter bay, the back of a knee wall, a rim joist, a crawl space wall. Many of the best attic jobs here use both, sealing the leaks and the rim with foam and then blowing cellulose to depth on the floor. For how that plays out attic by attic, read attic foam versus blown cellulose, and for the attic side of the work, see attic spray foam.