CentOs is free distribution of linux .
CentOs is based on Red Hat Enterprise Linux (RHEL)
What is RHEL ?
RHEL is a linux distribution developed by Red Hat Enterprises
RHEL is not free as like CentOs and paid support is available for RHEL.
At the same time Ubuntu is a free distribution of Debian.
What about Debian ?
Debian is a free Operating system with basic programs and has more than 43000 precompiled packages.
If you are going to decide between CentOs and ubuntu for your use , it will depend on :
The taks which you would like to achieve
You knowledge on linux distributions
In case of ubuntu , it has large set of features as compared with CentOs . Large group of users do use ubuntu for desktop environment but would prefer CentOs for their servers , as CentOs has cPanel support.
In CentOs versions are maintained for over 10 years (long release cycles) , but ubuntu is fast moving with 2 years releases . If you would like to have consistency , go for centOs
Other Linux distributions include :