The most common question we get on an estimate is some version of "should I just spray foam the whole thing?" Sometimes the answer is yes. More often, for an existing house with a normal attic, the answer is that blown-in insulation with proper air sealing gets you most of the benefit for a fraction of the cost.
Blown-in fiberglass or cellulose goes on the attic floor. It keeps the attic itself hot and unconditioned, which is fine as long as your ductwork and air handler aren't up there sweating through the summer. It's fast, it's inexpensive, and when it's blown to a measured depth over a properly sealed ceiling it performs exactly as rated.
Spray foam goes on the underside of the roof deck and brings the whole attic inside the thermal envelope. That's the right move when the HVAC equipment lives in the attic, when the ceiling is vaulted or finished and there's no floor to blow onto, or when you want the air-sealing effect that only foam provides. It costs more per square foot, and it's worth it in those situations.
The honest recommendation usually comes down to where your ducts are and what your attic looks like. We'll measure what's there, tell you which one fits, and put both numbers in writing if you want to compare.