If you expect to pay for Red Hat after a short test of its suitability, getting a 30 day free trial is a good idea.
But if you don't expect to pay soon, that 30-day trial is starting down a very wrong path. Get Centos instead.
That all assumes you asked the right question: What Linux to get in order to learn RHEL: Answer Centos.
If you should have asked what Linux to get in order to start learning Linux, the right answer is probably Ubuntu.
The only real differences between Centos and RHEL are license and support. My opinion on the choice between Centos and RHEL:
1) If you want to use RHEL in a production environment without learning it first, RHEL is a better choice than Centos.
2) If you already know RHEL well and want to use it in a production environment, Centos is slightly better. The cost of support is trivial. The advantage (to an experienced admin) of support is also trivial. The real difference appears when you want to cut over to a backup machine or make a big hardware upgrade or test a new approach on an extra machine before using that new approach in the live system. In all those cases, the inconvenience of working with licensed software ends up costing more than the license cost.
3) If you want to learn RHEL, Centos is better. Not just because it is free (zero $), but because it lets you experiment more without tripping over license issues.
If I were too good at convincing people of any of the above, I'd need to stop. Without enough people paying for RHEL, Centos wouldn't exist. But you will see many opinions different from mine, so I don't think I'm having a bad market impact.
Last edited by johnsfine; 03-01-2013 at 12:35 PM.