I use the C6 on my megatree. It does tend to fill in the gaps.
On my smaller tomato cage trees, I use the M6 LEDs and shorter strings. It looks OK that way.
On my roof, I've got the C7 replacement LED's to outline the roof.
I think each has it's place.
The C6 tends to spread out the light better than the M6. It doesn't necessarily look better one way or another.
If I was putting LED's on my arches or on wireframes, I'd probably vote for the M6's, since they wouldn't tangle as easily.