Most of what has been said is pretty much on the money.
The ATC's have a leaner sound compared to the Dyn's, and as a result will display a more open sound which will be more in line with higher end speakers. They do tend to prefer a little more driving than the Dyn's, and will work very well when placed against a rear wall as they're a sealed design. The Dyn's have a bassier, warmer sound, which will suit lower level listening better, and will be a little more kind to lean, harsh recordings. As they produce more bass, they will need some space between them and the back wall as they're a rear ported design.
Both are a little awkward for amps in their own way - the Dyn's because of their 4ohm load, and the SCM7's due to their 85dB sensitivity. Your Roksan should be fine though.
Personally I'd prefer the detail and edgier sound of the ATC's, but some may prefer the softer, sweeter sound the Dyn's will produce. Worth a side by side audition if you can. May also be worth trying the KEF XQ10's too, which are sort of between the two.