The short answer
Head spa vs scalp treatment starts with one shared fifty minutes.
Book the head spa. That is the answer whenever the question is asked in the abstract, and it stops being the answer the moment you can point at something specific on the skin under your hair — a patch that flakes, roots that are oily again by lunchtime, product you can feel with a fingernail.
A head spa is the reclining-basin session, sold in 45, 60, 70 and 90-minute tiers, and the plain definition of a head spa covers what fills each of them. Which tier you take is a calendar question rather than a scalp question: the tiers differ in length, not in what they are for.
A scalp treatment is not a longer version of that. It is a different line on the menu with a fixed shape — 80 minutes, always, of which the first 50 are a head spa and the last 30 are one targeted step: essence applied with a PDT scalp device, or 30 minutes of hydration. The scalp treatment page takes the second half apart properly.
So the two are not rivals in the way the search box implies. Fifty of the eighty minutes are the same fifty minutes, delivered by the same person at the same basin. What you are actually deciding is whether to buy the extra half hour on the end, and that decision has criteria you can check without asking anybody: what your scalp looks like when you part it, and how much of tonight you own. Mayo Clinic's write-up on relaxation techniques treats deliberate downtime as part of stress management, a fair way to think about the first fifty minutes either way.