I was using FreeBSD for ISP's but found that the linux kernel had better support for the hardware that was comming in.. We installed debian and redhat and slackware to replace it but gradually phased out he other linuxes because debian is better. It has a more intelligent layout and better package granularity (like slackware) but everything tends to be available and works out of the box (unlike slackware) I really like slackware but I wouldn't try and put it on an ISP because it is suited to people who like to tinker or hack.. not bog standard servers who's only role in life is news, apache, or radius services which they have to do well. The advantages of red-hat are it's user-friendly (more automated, less involving and less precise) administration and setup scripts.
Dave