Is it better to upgrade to Mavericks or to perform a clean install?
First, take a backup (using Time Machine or a cloning tool like Carbon Copy Cloner or SuperDuper! or Clonezilla) so that you can revert to it if at all something goes wrong and you're unable to salvage it through simple troubleshooting and fixes.

After that, just do a straight upgrade to OS X Mavericks. If you have several apps already installed, then an upgrade will save you a lot of time. The upgrade to OS X Mavericks will retain all your applications, settings and files (they won't be removed).

Also, a clean install is recommended only if you're having some issues in the system that you're unable to trace and resolve through other means. For every other case, a straight upgrade is reliable and quicker.

On the other hand, if you choose a clean install, then you would have to (follow this tedious and time consuming process):


Backup your system (using Time Machine or a cloning tool like Carbon Copy Cloner or SuperDuper! or Clonezilla)
Install OS X Mavericks
Configure all the settings for OS X Mavericks as if you were setting up a new system
Install all your applications afresh
Configure each of your applications according to how you'd like
Copy all your files and folders from the backup