These speakers are both about the same sensitivity - 84-85db.
Since the tweeters are the same, you can expect the same sound quality on the top end.
The standard 1801b's will not play as deep as the SongBirds and will be slightly less flexible in terms of room placement since they are a ported speaker, not a transmission line speaker. They will also not play quite as deep as the SongBirds, but will handle more power. In other words, they can play louder befpre reaching the maximum excursion of the driver. The 1801's will be more detailed in the midrange due to the stiffer, lower-mass magnesium cone of the W18.
Now, if you compare the SongBirds to the 1801-TL's, that is a different story. The 1801-TL's will be as easy to place in the room as the SongBirds. They will be slightly more detailed in the midrange, will play deeper and will handle more power as well.
If the room size is on the small side and you will be using them at a moderate volume level, the SongBird's will perform quite well. But if given a choice (and cost was not a consideration) the 1801-TL's would be the best choice over all.
I should add that we do happen to have one nice curly cherry pair of 1801-TL's available for immediate delivery.
I hope that helps.
- Jim