Here's my two cents.
Joomla 2.5.X is no longer supported. This means that security issues and other such vital stuff will no longer be addressed.
Since Joomla is in the Opensource domain, every one knows its structure and the assets that Joomla uses.
Hence, whenever a security hole is identified it needs to be plugged immediately. which in the case of Joomla 2.5.X won't happen,
Over time the Joomla sites built on version J2.5.X will becomes very vulnerable.
My advice upgrade to Joomla 3.X.X as soon as possible.
This is easier said than done. Do be careful when migrating Users from J2.5.X to J3.X.X. While the talk is that is seamless that's regretfully not true. The xx_user table structure in J3.X.X has changed, so you will have to manually export your J2.5.X User data, edit the data carefully to map to the J3.X.X and then import the user table data in the Joomla 3.X.X installation.
While this is not rocket science, its tedious and needs to be carefully checked and cross checked.
Also, make a list of all the plugins you've used on the J2.5.X site and figure out which are compatible with J3.X.X which are not.
You are going to have to find equivalent plugins for those that are not compatible and then figure out how they have to be used.
NOTE: Please make really sure that you make a complete backup, folders, files, and database, before attempting an upgrade just in case. Joomla can be quite cussed at times.
All the very best.