I think Rarity's personality is a lot more bearable than Rainbow Dashes. When Rainbow Dash personality is over the top, it comes off as arrogant and mean. When Rarity is over the top it's more comical and drama-queenish. I just like comical and drama-queeny more.
Also after May the Best Pet Win, my opinion on Rainbow dash changed a lot. She was so hypocritical(imo) in that episode. She wanted a pet that was loyal, and yet had no problem abandoning, and being unfaithful to the animals that were trying to gain her friendship. Then she had the nerve to put that poor falcon down at the end, for leaving her in a race as though it was somehow the huge jerk in the episode.
That entire episode just left a sour taste in my mouth for Rainbow Dash(and this is coming from someone who liked the hated Mystery-Mare do Well). It sort of improved again though after Wonderbolts Academy, but not enough to dethrone Queen Rarity.