Debian stable seems a better pick right now, because it is newer than CentOS-5. This in itself is temporary, until CentOS-6 is released within the next 10 months and then CentOS-6 will be for some time newer than Debian Stable.
On the other hand, Debian Stable only gets security updates until one year after the next Debian Stable. Which effectively means you have to upgrade in that one year. Which is vastly different than with CentOS, where you know in advance you can run it with security updates up to 7 years after General Availability. For CentOS-5 that means until 2014 ! For Debian Stable it is hard to tell, you only know it exactly one year in advance :-D
It could be in 18 months, if Debian is able to follow a strict release schedule. Or it could be in another 3 or 4 years ? In both cases it is worse than what CentOS offers, even if that means that you get something older than Debian Stable at *this* moment in time.