Can a chain-lock be better against cutting than a U-lock?
As for the Kryptonite, cutting one side an bending is certainly possible, but in reality if you can cut one side, you use the same tools to cut the other. The tools needed to bend the link after cutting one side are almost certainly not portable, and why would you carry two tools when one will do.

In comparison to the D-lock--if you can cut though 18mm, you can cut through 20. It will make no difference. The main difference is a D-lock is susceptible to different tools to a chain. For real security - use both. 

The reality is if they want it, it's theirs, and there's sweet nothing you can do about it. 
The job of your lock is to make your bike less desirable than the one next to it. If you bike is a $10K dream machine, that's going to be a tough job--you will need so much heavy metal it will weight more than the cheapest supermarket hack, and take so long to secure the time saving you get riding it is lost at each end of the journey. If its a $100 hack, then any half decent lock will be good enough.  

The best you can do in reality is "enough to satisfy your insurance company".