I have listened to both for only a short time, maybe 3 hours on the Open Flows and double that on the C Flows but already I feel I can recommend you both get the Open Flows if isolation is not a problem. Isolation is a problem for me at times so I needed a closed headphone too and for those times, I would NOT prefer the Open Flows over the C Flows because of the leakage and isolation, which would ruin the experience.
However, the Open Flows, when amped up and in a quiet environment are superior in sound to the C Flows. They are more open (duh, of course) airy, textured and 3 dimensional in the sound. With live recordings you feel more like you're in the club with the band. I say club because the sound stage feels "intimate" like a small Blues club opposed to Shea Stadium. With the C Flows you're always kind of reminded that it's a headphone but with the open Flows you are transported more easily into the music scene. Both are detailed, go deep into the music without getting clinical but with the Open Flows you get more separation lending to a more 3D effect to the music which makes it sound more natural.
The Open Flows are a bit brighter, but not in a shrill way. Rather in a way that fills out treble music better than the C Flows do. I actually think the bass on the Open Flows is a tad more realistic too. I say this of course from my own perspective, realizing that everyone hears differently and may not agree with how I hear things. I also have a bit of high frequency hearing loss so don't trust me if I say something isn't bright because I may not be hearing the upper registers the way a person with normal hearing would. I've only lost about 25% so it's not that bad, and I'm getting kind of old too so I guess it goes with the territory.
When I put on the Open Flows I'm transported and moved by the music. I don't get quite as pulled in with the C Flows, well sometimes I do, depends, and the overall caveat here is that the magic of the Open Flows is lost if the environment isn't quiet, in which case the C Flows will sound better in comparison because you're getting more of the music in those situations and not having to deal with noise pollution.
As Tyll Hertsen says, the minute you put these sorts of thoughts into print it makes the issues seem larger than they really are and I feel the same here. The sonic quality between the C and Open Flow is closer than my descriptions probably make you feel about it. There is a difference though and if I had to keep just one I'd keep the Open Flow and use my Audeze Sines when I needed closed HPs. I'm fortunate in that I can keep both and I shall.
Some other thoughts: Both of these headphones are crazy-easy to drive. They work much better than the specs would lead you to believe. For example, my Audeze Sines, on paper, are more efficient by a large margin and yet in practice it takes more power to get them to the same listening level as the Open Flows. The C Flows take nearly the same power but even so, they are still slightly easier to drive than the Sines. So, don't trust the specs necessarily when it comes to how an HP will perform from an IOS, Android or computer. Both Flows will get nice 'n loud from any of these. Amped they sound spectacular and unamped they still sound really good.
