Esta is restricted in his choice of conversation branches by not quite wanting to give away what he and Mariona used to do for a living.
The fact is, most cleric spells are not really what you'd call personal convenience spells. It is self-obsessed wizards who are free to invent the likes of Age Resistance. Cleric spells are god-made to be actually important: combat spells, buffs, military infrastructure like Scry, civilizational infrastructure like Create Water or Stone Shape.
The spell that Esta has the most personal fun with, when he is relaxing in some young lady's unconsenting company, is the spell Bestow Curse. You can do so many things if you invest some time and effort and Spellcraft in realizing the full flexibility of Bestow Curse. (It's not even wasted effort for someday ruling the Church. Rugatonn's supreme expertise in curses, used for the likes of 'cursing Chelish workers abroad to be unable to act to escape Cheliax', is arguably more valuable to the Chelish state than her ninth-circle spells.) Esta thinks he maybe shouldn't mention that part either.
On Mariona's literal words as to what spell merely being able to cast brings forth the greatest change in day to day life, there is a single very obvious answer: The defining spell of 4th-circle: the spell whose willing casting gets you told the real truths about the Church*, promoted to the true inner ring** and all the wealth and power that brings for everyday life: that spell whose unforced casting is considered nearly as good as a wizard's soul-sale, in terms of making its caster to know theirself for damned in a way that is nearly impossible to ever atone for, and with correspondingly assured loyalties: Malediction.
(*) Esta does not know the real truths about the Church.
(**) The actual lnner Ring is "Rugatonn, Gorthoklek, and Lrilatha", but 4th circles and above believe themselves to be in it.
"I suppose [Word of Recall]," Esta replies. "It reduces the number of overnight missions, and gets you sent out on other missions where you can save a 7th-circle spell on the [Greater Teleport] going back. So there's a marked difference in how you spend your days, afterward."
"Though I might just be thinking of that one because it's my own most recent change. Other clerics might name the [Guidance] cantrip, which is a little boost to any skill that requires quick decisions, but that trades off against other important cantrips like [Detect Magic] or [Resistance]."