Because of the nature of Debian, Lazarus is always a version or two behind in the Debian repositories. They are also broken up into multiple packages.
Installing by means of Laz built debs may work OK, but they are built on Ubuntu instead of Debian, AFAIK. That has the potential of being problematic. Installation of the RPMs via alien works well and avoids Unbuntu.
However, both of the above methods can result in naming conflicts between the debian repo's packages and the laz built packages. That can cause real problems during a distribution upgrade unless the packages are locked or pinned, which in itself can be a headache.
A better way, in my opinion, is to build from source and install under the user's home directory.
FPC installed from the Debian repositories does not tend to have these issues, but that might change with the release of 3.0.