I have been in a CB6k for over a year now, and am moving on up. Currently use a 45mm base ring. Will that be the same with the Oval ring compared to the round?
When I went from the CBx 6000 to the Holy Trainer I wondered the same thing I ordered both rings with the HT and found the smaller one more comfortable which was about the same size as the CBX. The oval shaped ring doesn't move around as much as the round ring did. This made it more comfortable at least for me anyway. I've ordered steel cages with both round and oval rings and the oval is my personal choice and what I wear now.
the oval form takes care of the "seam" running from your scrotum to your anus, so usually a smaller ring-size will be allowed
You have to be careful with sizing oval as the penis is not circular, nor oval, but sort of ovally circular, some with a triangular tendency. Just look at this lot for its variability: https://www.google.co.uk/search?q=cross+section+of+penis&tbm=isch An oval ring fits better for some with the long axis up-down (or front-back, whichever way you look at it!) and some find that the larger dimension is side to side. You can do a quick check by using two pencils or similar to measure the thickness front-back and side-side at the root of your penis. Most oval device rings are built as default with the long axis up-down, but a custom builder should be able to cater for either option.