THE K3 VISA
The K3 visa is kind of like a fiance petition, filed at the same time as the spousal petition, to allow the applicant to enter the US while the spousal visa is still processing. The advantage is this allows the spouse to enter the US a few months sooner than would have been the case without. The disadvantage, of course, is it costs considerably more. The reason it costs more is, by entering on the K3, it is a temporary visa which allows the spouse to stay until processing of the spousal is completed. This means that after the spousal is completed, you must adjust status to get the green card, just like a fiance visa, to the tune of $1225. Whereas, if the spouse enters on the spousal visa alone without including the K3, The green card is already processed and will arrive a few weeks after entry.