I have pondering this for awhile, and I think both the ring and the cage are bad choices for MMA, in that they both benefit and detract fighters. I feel that, while good matches can obviously happen in both atmospheres, there should be little outside influence to potentially effect the result of matches.
With that said, I think the WCL has the right idea when it comes to combat. It is completely circular with no posts or cage mesh to impede on a match's progress. The only thing I would change about it is make the mat white and the outside lip also a light shade, and also make it larger.