What does harder mean?
At this point you need to just pick one. OpenBSD has the most secure default installation out there. It isn't the easiest to work with for those new to unix but it isn't that much harder than linux to learn. If you're planning on using the command line, then it really isn't harder at all.
If your hardware is supported under OpenBSD then get that. No offense intended whatsoever
, but based on the questions you are asking, I'd say learning to secure a RH (or other full-featured distro) linux box would take the same effort and research for you as installing and configuring ftp (or any other basic server, really) on OpenBSD.
OpenBSD (or FreeBSD really) on public servers, linux on internal servers (database, intranet, file/print, etc) and desktops.