I don't have any great "concrete" reasons and even if I did, they would apply to me, not to your individual systems. It's just that as a general rule, I think that the pairs of releases, which you could argue are SQL Server 9.0 and 9.5, you'd be better off with the new version.

My guess is that there is a team working on SS2K8 and a separate team doing patches/SPs for SS2K5. Some of the stuff that gets fixed moves over to SS2K8, but I'd agree that it's probably a stability v9.5, but with new features, like the policy based management, it's pushed as a v10.