Are you saying that the gaps between Ubuntu LTS 6.06 and Ubuntu LTS 8.04 are smaller than the gaps between RHEL 4 and RHEL 5? They both have about a 2 year gap between releases.
Since the RHEL and Ubuntu LTS release calendars don't sync up, they are essentially staggered about a year apart, there is no direct technology comparison. RHEL 4 is a year older than Ubuntu LTS 6.06 which is a year older than RHEL 5 which is a year older than Ubuntu LTS 8.04. All of them will become stagnant overtime.