I have both the C1 and the S25 on hand right now, and frankly, I'm having a difficult time deciding which speaker I prefer.
The C1 is definitely the smoother of the two. The S25 can sound a bit forward and therefore slightly fatiguing on certain material that is not well recorded. However, this slightly forward character can make the S25 more enjoyable for rock and electronic music. It really gets your blood pumping. The S25 also has much more powerful bass on its own, and can be played loudly without having to worry about bottoming out the woofers.
I feel that the C1 is the more accurate speaker. The treble is absolutely gorgeous, and the mids are pure and uncolored. Really, the only area where the S25 beats the C1 is in the bass, in both extension and power. I don't know how else to describe the C1 other than I feel that it is a very accurate speaker. The fact that I can't think of any adjectives to describe its sound is indicative of this trait.
I'll try to sum up quickly and simply. If you like to "rock out" with rock or dance and/or any other type of hard-hitting, dynamic, rythm driven music, and have a larger size room, the S25 is probably the better choice.
If you have a smaller sized room and you like acoustic music, vocals, string quartets, piano etc., and are looking for as accurate reproduction as possible from a monitor speaker, then the C1 is probably the better choice.
I could live happily with either one. Both speakers have that beguiling quality that makes Dynaudios great.
Here is my associated equipment list:
Denon DVD-5900 (transport)
Proceed AVP2 (DAC and preamp)
Simaudio W-3 (amp)