I've listened to both extensively, and I've had the Song 3 Encores in my room. (a friends pair).
I also really like both ATC and Salk Speakers. I've not owned ATC's, though I would consider them.
I've owned Salk SS8's, and now own Salk SS12's.
Both are going to measure pretty darn flat and neutral.
I feel the Encores will actually play a bit lower and give you a little more bottom end. They also MIGHT have a little more top end sparkle with that Be tweeter (I normally do not like Be tweeters, but Jim Salk, along with Revel are the two companies that I've heard where they managed to tame the brightness).
Midrange - both are quite good and accurate. Neither speaker is going to be super forgiving.
Ironically, they are more alike than different IMO. If you don't have/use a subwoofer, I would point towards the Salks. Both have really nice cabinets, though with Salk you will get a custom cabinet that you can pick.
My note would be the following, and as mentioned before. IF I were to get ATC's, I really think their active speakers would be the way to go. I've heard the 50's at a show both active and passive with a nice amp. The actives had a crazy amount of "jump". I really liked them.
I like bass and I do not like subwoofers with 2 channel if I can avoid it.
If I could afford them, I would be really tempeted to get the 100's or 150's in active form.
If you like changing and trying new amps and pre-amp's out, want a custom finish etc I would highly recommend Salk.
IF you want active speakers, it's hard to go wrong with the ATC's.