The major factors that should affect your decision as to which version to install include:
What degree of stability does your installation need?
How much effort do you wish to devote to upgrading?
How long do you intend to stay on a particular version between upgrades?
How important is security to your installation?
Will you be installing from source, or from binaries?
Are you willing to help participate in the FreeBSD development process?
Here are some rough guidelines to help you in your decision:
If you have a short-term need, would benefit from the highest degree of stability currently available, and are not able to devote many resources to upgrading, then you will probably want to install the latest STABLE minor release and remain with that. Depending on your security needs, you may or may not wish to track changes to that branch as they are released.
If you have a short-term need, and feature completeness or the best levels of security are most important to you, and you are willing to spend time upgrading, you may wish to track the latest STABLE branch.
If you are not immediately planning on going into production, you are willing to work through a certain number of problems, and a new major release is upcoming within the next few months, you may wish to investigate installing that branch to help the project to test it and stabilize it to make it the best release possible in the medium to long term.
Only if you are willing to install from source, and spend time debugging problems with the base system and follow up with problem reports, and track the mailing list that deals with such issues, should you consider tracking HEAD.