i would vote other if it was an option. it wasn't, so i voted no.4 though i don't agree with it fully. firstly, if they were supposed to already live together, they wouldn't live in sara's apartament, but grissom's townhouse, which is bigger. another thing, i believe grissom's ego, wouldn't let him move into sara's place. so that bedroom decor, is quite confusing. we don't know this warm side of grissom at home. heck, we don't know any of his sides out of work.
about the option i voted, i don't agree, that he's so cold and without soul to live in a house with empty walls and one chair. yes, we saw his house back is season one. but we all know he evolved so much since then. what would express his present style ... i would mention the straw hat, which is very cheerful, and reveals the funny, creative style of the new Grissom.
to get it all together: i think it's Grissom's house, which he was redecorating step by step throughout those years (and mostly since the straw hat thing), and when sara came to his life, she added a couple of things to it too.
ETA: just had a thought, can anybody imagine Grissom beardless with a straw hat on? I CAN'T! please, Billy don't do it!