I wasn't necessarily meaning musically, but content-wise.
Xiu Xiu are generally more disturbed, full of moodswing outbursts, and some very open and detailed descriptions of mental health problems. The Red House Painters are full of melancholic stories, often to the point of being so personal and honest that it's frankly uncomfortable, especially earlier on in their career ("Katy Song", "Michael", "Mother", "Evil", "Medicine Bottle"). Another possible point of comparison would be Smog, which is full of melancholic stories, but often with a bitterness or a black humour, whether it be sarcastic (cf. "Dress Sexy At My Funeral"), or downright disturbing (such as when he makes a doll out of an ex's underwear in "All Your Women Things" or when he interrupts a very bleak, monotone, droning song about a Hannibal Lecter-esque female character in "Real Live Dress" to sing a few lines of "Baby Got Back" and you can almost see the sick smile on his face).
For me, the RHPs are one of the best bands of all time, but then I absolutely adore most "slowcore" bands like them, Codeine, Slint, Low...