Stan, Ive listened to both when I bought my C5s. The differences between the two were small. The C7 seemed to sound a tad more brighter overall. Perhaps due to the smaller woofer configuration. They were to close in sound to warrant another two hundred bucks for me anyway.
That said I LOVE the C5s, I find myself sneaking away from the wife and kids just to turn em on for 10 minutes here and there
I doubt if there's much difference between the two, except that the C-7's have lower extension. (I have a pair of C-3's and really like them.) If you can afford them, I'd go for the C-7's.