I don't think it's true for either of them. Rarity is a bit stuck up, but she is there for her friends, even when it's more out of vanity's sake. She first meets twilight and instantly wanted to help fix her hair. I'd say she's by far the most empathic of the group. She' might not be at the top pf my list, but she's a good character.
As for Dash, she's confident to a fault, but she never lets it get in the way of her friends. She'll always be there when they need her, she just doesn't have the empathy of Rarity.
See, after I defend Rarity you go and post a big fat dirty lie like that...