As you will here in the responses soon to come, there is no worse or better. It is purely subjectve, based on your ears and taste for sound.
The 170's are described as extremely transparent, meaning they do not color the sound at all. They have a 6 1/2" woofer versus a 5 1/4" for the atom. So, you are getting a bit bigger speaker with the ascends. That does not always mean better. It may mean lower bass extension. I am uncertain how transparent the atoms are. I am certain you will get responses on that as there are a lot of people who have and like paradigm here.
I would recommend ordering the 170's to demo and go to a paradigm shop to demo the atoms. They may even let you borrow the atoms, so you can do a side by side with your own electronics. Another point is that ascend has no middle man markup, no dealers, so more of the cash can go into the speakers. Paradigm is sold in high end shops, and they have to get paid too.
Good luck!
Pat