Some reasons to use the official release instead of a distro release:
- Distro packages often have bugs introduced during packaging.
- Kovid (author of calibre) won't support distro packages.
- You won't have access to bug fixes.
- You won't have access to features added after the distro release.
Take a look at
calibre's change log to get an idea of how fast calibre moves and what you will be missing by sticking on 2.5 (current release is 2.20).
On the other hand, if that release works for you and you aren't interested in anything in a later release then have at it.