Man, I've been looking into which one to pick as my next free plugin, assuming we get one more.
I'm in love with both of these.
The 670 is perhaps the smoothest compressor plugin I've used. I've been testing all of them and other plugins (other makes) and the 670 has a very natural sound to it. Using on the master, it literally takes a dead recording and makes it sound so natural and alive. My only caveat is I can't seem to get too much volume from it w/o distortion. But the little bit it gives is practically flawless.
On the other hand, The One module, seems to be a happy middle ground. While not quite as natural sounding as the 670, it provides a wider, deeper bodied tone and provides the volume increase w/o distorting.
No matter what, I love them both. IK Mult. is doing such a good job with these plugins.
Well done IKM!