Hmmm... really, when you get down to it, Twilight and Rarity really do have similar personalities. They both have similar work ethics. (That whole "I'll sleep when it's done" thing. Applejack does that, too.) Really, the main difference is in how they regard a stranger. Rarity is grace, poise, and (above all) generosity. The model hostess. As long as someone at least makes an effort to not be a boor, Rarity will do whatever she can to accommodate them. Twilight, on the other hand, is the pragmatic academic - friendly, polite, yet aloof and skeptical. She only really opens up with her close friends, and even then it can be an awkward experience. Even if she doesn't realize herself that its awkward. (OMG - positive feedback loop of awkward!)
Does that help any?
Does that help any?