Defense versus Attack
^^^This
The Gripping Hand
On one hand, a 4*+15 dispeller is a 4*+15 dispeller.
On the other hand, dispellers only are useful if the enemy has buffs.
On the gripping hand, when you need a blue 4*+20 Paladin / 5* 4.80 dispeller or a buff thief, you need a blue dispeller / buff thief ( looking at you rare red tiger titan ).
So once again it comes down to what is in your hero roster.
Be flexible
I was going to level Sorcerer Sabina to 4*+1 first, but Devs added emblems to legendary tier completion rewards for challenge events. So I broke my rule of same color leveling. Off color, 1 day, power leveling Proteus from 1/8 4* 1.25 to 4*+3 for the challenge event was painful for me, but worth it ( the loss of -20% Hero XP is balanced by the 3*/ 4* battle items I can save in Legendary tier and use for the 8 of 10 class quests Proteus is banned from entering ).
Even if you plan to take Sonya to 4*+20 ( Paladins are one of the few classes that get a huge benefit from the last node ), your can always stop spending emblems on Sonya, or even reset her class as others have mentioned.
Reset costs
While I very strongly disagree with Devs over their implementation of emblem resets, it is what it is.
But you CAN reset emblems, so at least take Sonya to 4*+1. My Scarlett 4*+3 evaded a Sartana Deathstrike, which my Rigard cleansed, and I laughed out loud. Sonya 4*+1 triggering Protect 1 or 2 turns before a Deathstrike would significantly reduce the initial damage ( +25% defense for 2 turns can be huge )
Sonya 4*+1 may not get a lot of +Defense, but if you play Empires long enough, it will happen, and more than once. One of the brilliant aspects of talents is the proc rate changes, but the benefit is fixed ( teaching more of the player base about RNG and adding more fun and more players likely to go “Fudge, I am going to try attacking that 5* defense anyway” ).