R8 provides more defenses (the bonus Atk/Def levels gained make a big difference, not to mention the Phys reduc %) and more offensive capability. R8 that has some investment into it can be far more OP than Dragon (an example of this is getting channelling on all R8 gears, as opposed to having limited channelling on Dragon). In your case, clerics (supports in general) tend to go with a mixture of HA/R8 (usually HA top and rest R8) as this provides more defenses for the cleric and allows them to properly support their team while in PK. I would recommend you look into getting some channelling on your gears or +Pdef/phys dmg reduce % in order to increase your survival.