If your server is safe does not really has sth. in common with the distro, more with the configuration and administration of the machine, from basic stuff like apache and their modules config, postfix config (avoid beeing an open relay) etc.
Anyway, why do you want to use ubuntu on a server? As I far as I know *buntu has the same goal like xandros etc. to have a comfortable GUI etc. but on the console they are mostly the same... so I would use Debian (on which *buntu is based on) or CentOS (better to be compared with your current RHEL)