I apologize, but I still do not see your point.

The 2 options for for Surtr is either 40% ATK or 20% ATK + 20% ASPD from the guard equipment.

My statement

Because she already receives a large attack buff, you want to build up ASPD. 

implies the use of equipment B to build the 100% ATK + 100% ASPD.

Are you getting confused by the word "buff?" The "buff" I was referring to was Surtr's skill which can increase your attack by +240~330% which is why it's important to build equipment B. It should be noted that both the equipment and her skill are additive buffs and therefore, will not effect each other. Only her base stat will be calculated when determining the final attack increase for both.