Short story. I'd go with air-sealing the attic floor and more blow-in insulation. Make sure the air-sealing is done well - it's important.

Long story. Both methods work, but whether to vent or not is a topic that causes arguments, even among experts (and I'm not an expert). I go with the conventional approach because you'll get payback within your lifetime and it's straight-forward work any insulation contractor can do. That said, there doesn't seem to be any doubt unvented attics are the Rolls Royce of attic insulation, but they're expensive, difficult to very difficult to implement if the roof isn't simple (dormers, hips, valleys, etc), and require knowledgeable, conscientious installers.

Some of the answer depends on where you are locatedl=.
- If you air seal and add insulation to the attic floor, and then increase the attic ventilation as much as possible (for example, continuous soffit vents and a ridge vent) it will be the most effective for a hot climate where you are trying to reduce heat gain from the sun. You are effectively creating a vented umbrella over the house and keeping most of the heat away.
- If your primary need is to keep out the cold, I'm not sure which would work better. However, good ventilation is also needed in a cold climate to keep down condensation, so I think sealing and adding attic floor insulation would also be effective.

Bruce

more info please.

what climate?

what type of insulation do you currently have?

any recessed lights?

where is heating equipment & ductwork currently
located?

what is roof pitch?
simple roofline or complicated?

cheapest is not always best.
nor is most expensive always best.

adding insulation without air sealing between
attic & will lower performance of insulation.
creating as perfect of an air barrier at the
attic floor/ceiling plane is a very cost effective
upgrade. then attic temps, insulation particles,
& humidity stay in attic.

we do a lot of unvented attics here in La. because
we do stupid things like putting ductwork &
equipment in the attic. then add dozens of recessed
lights that aren't air tight, cut oversized holes in
ceiling for the can lights, supply boxes for hvac,
bath fans, stove vents etc.

foaming the roofline takes these multitude of
building failures and makes them less of an issue because
the air barrier & thermal barrier of the foam insulation
has now been moved from the attic floor to the
roofline. when this is done the holes in the
attic floor no longer suck air from attic into the house.

take time to answer some questions &
educate yourself before just adding insulation.
read some of the threads about cellulose & 'dust'
issues.

foam air seals & insulates as one install, when properly
done. other insulations slow air movement, but do not
stop it.

best of luck.

Bottom line, you don't want to pay money to heat and cool the additoinal space of the attic. Insulate the attic floor and seal it off from the house.

If you insulate the attic ceiling, even with the sealing off the outside air, the result is more volume of space within your building envelope that must be heated and cooled.