Dad’s short answer: The brush roll, most of the time. Suction moves air; the brush agitates the surface so there's something for the air to carry. On carpet especially, a great brush head with moderate suction beats big suction numbers with a poor head.

Why agitation beats airflow on carpet

Dirt in carpet isn't sitting on top waiting to be lifted. It's worked down between the fibers and held there by friction. Suction alone pulls at it and mostly loses. A brush roll physically disturbs the fiber and releases it, and then airflow carries it away.

That's why a machine with a well-designed powered head outcleans one with a bigger motor and a passive nozzle. It's also why every serious cordless and corded vacuum has a motorized head now, and why handhelds without one disappoint on upholstery.

What to look at instead of the suction number

Whether the head is motorized at all — and whether the upholstery tool is powered too, not just the floor head.

Whether there are separate rollers for carpet and hard floor, or one dual-surface roller that handles both. The dual-roller designs on better machines genuinely work and save you swapping heads.

Whether the roller is anti-tangle, which is a big enough question that it has its own page.

And whether the machine detects the surface and adjusts. That sounds like marketing, and on the better units it actually works — several machines I've scored highest ramp power up on carpet and ease off on hard floor, which saves battery on cordless and noise on corded.

Dad’s Take: Look at the brush head before you look at the suction spec. It's doing most of the work either way.