1. RHEL support is what you pay for in time options, actual quality doesn't vary. Basically (iirc) it's 1. web tickets only, 2. phone office-hours, 3. phone 24/7. It's the same guys at the other end...
2. Centos is a free version of RHEL (sans proprietary logs etc) and no support.
Very stable and updates for 7 yrs
http://www.redhat.com/security/updates/errata/