First you need to distinguish paid vs. non-paid variants.
Suse has SLES and IBM has RHEL. I am not in the position to compare them.
The non-paid variants are OpenSUSE vs. CentOS. One big difference there is the support life.
CentOS 6 with an ancient 2.6.32 kernel and upstart came out 2011 and is still supported. OpenSUSE releases are supported ~ 1.5 years. There are different opinions whether upgrading from one OpenSUSE release to the next one should be done or not. I typically prefer new installations because they result in cleaner systems. But I also upgraded once an it was painless.
Suse has SLES and IBM has RHEL. I am not in the position to compare them.
The non-paid variants are OpenSUSE vs. CentOS. One big difference there is the support life. CentOS 6 with an ancient 2.6.32 kernel and upstart came out 2011 and is still supported. OpenSUSE releases are supported ~ 1.5 years. There are different opinions whether upgrading from one OpenSUSE release to the next one should be done or not. I typically prefer new installations because they result in cleaner systems. But I also upgraded once an it was painless.