i haven't tried karmic koala, but the reason i use 9.04 is because its the only ubuntu available to select, other than 8.04 that was once available, but 8.04 lacked allot of support for stuff unless you went into back-ports.
I currently have Ubuntu 9.04 on my PC. I had the a similar experience with Ubuntu 9.10. When 9.10 was released I was hoping for an improved better running system but found it to be a buggy mess. Fortunately I tested it on seldom used machine before putting it on my primary PC.
When I started setting up my VPS some time ago I was tempted to go with Ubuntu because I was familar with it, but, based on my research, it appeared that Centos was going to be easier to set up, use, and manage. I think Centos interacts better with Webmin and Virtualmin - both of which I use for GUI on the server. So far I have been very pleased with Centos.