I created a pretty in depth thread regarding the competition werkes exhaust. Most of it is in the group buy section. Also there is another member that has written their review of this exhaust. I dint know anything about the Ixil though. Either way I suggest you use the search function as the info is out there.
I love the way the CW exhaust looks and have only heard the youtube stuff on it.
The Ixill looks sooo freakin ugly but sounds incredible!
They are about the same price so that is not a factor. I was thinking about both as well but might try and fab my old slip on from my 06 600RR since that and the D675 have similar exhaust (I mean they both have undertail, if it works right i dont know).