You have a point on some aspects, Diego, but I'm afraid I'm still not sold on the proper adjutication of this spell's level. Especially on the magus case. Let me explain it as a general case first and then in the case of the magus ( and somewhat less, the Inquisitor's). As a generic case, comparing it between both spells (a touch, quickened spell in a 6th level spell slot - to balance out spell slots and kind of action- Vs decapitate) you have to consider:
Disintegrate: sure, if you have taken Close Range. AFIK a magus hasn't so many nice ray spell to make that a mandatory choice unless you build your character around it.
I concur that there is no so many ray spells to make close range an enticing option at low and middle levels, per se. But once you get the disintegrate spell, well, let's say you and almost every magus in the world would be very happy to take "close range" as your next Magus arcana.
Note that decapitate can be cast in addition to disintegrate, you cast disintegrate as part of spell combat, decapitation as a reaction after scoring a critical.
At first I thought that this might be the one and only reason for me to agree on setting decapitate as a 6th level spell: both competing for the top level spell slots to avoid such a combo. But since spellstrike clearly says that spell crit is always x2, disintegrate makes you dust no matter if you saved or not when you have zero or less hps left... I still can't see the need to spend high level spell in such conditions ( the extra 4d6 not as base, multipliable-on-a-crit, damage).
BTW, on a critical disintegrate is a minimum of 64d6. If I am fighting a magus and have access to greater spell immunity I will select it as one of the warded spells.
Ups, sorry, I miscalculated that. And at these levels, it's a very sensible thing to do to ward yourself against disintegrate, in any case.
"Finger of death or slay living" are standard action casting time spells. Comparing them to a immediate action spell is comparing two widely different things.
No, no, no... I'm afraid that you might have misunderstood me. What I was trying to say is that Decapitate , as a 5th level spell is quite more interesting to them than to magus ( and, obviously, wizards ), specially since they don't have spells of the likings of slay living or finger of death: deadly and dramatic. Again, my bad, sorry.