Ubuntu server is well patched so its quite secure and stable, so is CentOS but it comes with older kernel so new hardware could be an issue.
Hard to say what's the most stable, but off the cuff I'd say CentOS. But yeah, hardware support on it is an issue. Ubuntu 10.04 LTS is quite good and stable in my experience, and I would definitely recommend it for newer hardware.
I dont know models from my head but its dual core Intel pentium vs ASUS board (2 year old). Everything integrated.
Serverwatch reviewed Ubuntu LTS server here http://www.serverwatch.com/sreviews/article.php/3885826/Ubuntu-LTS-1004-a-Linux-OS-at-Its-Best.htm