This is actually a very astute question, as I really like both of these speakers, and they both share a transmission line cabinet, and have their drivers arranged in an MTM layout. Though the Adagios cost a bit more than twice as much, they are actually quite similar in design. SongTowers are rear ported, the Adagios, front. This in itself may be a factor if proximity to the back wall is a problem for your space. Then again Jim may be able to come up with a front ported version of the ST for you if you were to ask (then again maybe not, since the cabinet itself acts as part of the speaker a front port may entail a complete re-design...) Still something to consider with these two models.
Let me start by saying that I heard these two speakers in vastly different rooms, driven by vastly different electronics so YMMV...
The Adagios are extremely fast, and tight. Across the board. To my ears the ribbon tweeters give a more extended high end, and the mid-bass drivers used in the Adagios extend lower than those in the SongTower. The SongTower's however, win the prize in the midrange! Both are very musical speakers and performed very well with the music that I used to audition them. If you like heavily percussive music I would pick the Adagios, hands down. If you are more into vocalists, you might prefer the SongTower. I think the SongTower's are a better value than the Adagio's, but then again if cost is not an issue...
Even though they are a similar design, and are both quite neutral and uncolored in sound, they each have their own character. Adagio's are the more the dynamic speaker, the ST's are the smoother.
In the end it would come down to your own personal preference, and budget. Both are exceptional and could make even the pickiest guy(or gal) happy for a long time to come. If you are interested in the Adagio's I would suggest looking on Audiogon, as I have seen a few pairs up for sale there for a decent price. Funny thing about that is that more than one person said that they picked up a pair of Adagio's to listen to until their uber-expensive speakers arrived...
One last note on the two is the finish. The Salks you get a nearly infinite selection since each is custom built to order. The Adagios only offer four available finishes. Both speakers look great in person, and have a similar stature.
Both Robert Lee, and Jim Salk are great guys to deal with, very friendly and helpful. I get the impression from both that they are genuinely happy to help you out.
If you have the chance to listen to both speakers, please do so and let the rest of us know what you thought. Anybody else had a chance to hear the two? Care to share your opinions?
Hope that helps you out.