just go for ubuntu... if u working on rAthena using git diff command, u will see the diff more clearly on ubuntu.. the diff on ubuntu comes with green & red color which make u easier to notice the changes.. see the below screenshot... if u really optimize & utilize git diff & git status command, u will loves using ubuntu..
there are some mores advantages using ubuntu rather than on centos... customization & modification of apache webserver (adding virtual server) & phpmyadmin.. but for these 2, i don't think u will go that far for the customization & modification..
i'm prefer ubuntu not because of its stability reason, but bcoz it is more on handy & easy to use & manage (if u loves doing lots of customization & modification). if just doing default setup, i think it shouldn't be a problem (especially for beginners who are just to get things work around)