Which should I choose to install CIAO: conda or ciao-install?

In CIAO 4.17 you can install CIAO the "old" way, that is with ciao-install, or with the conda package manager (which was first introduced in CIAO 4.12). There should be no significant difference when using CIAO installed in either manner (the CXC can not guarantee that the results will be exactly the same due to differences in underlying libraries and compilers between the two systems), so why do we have two different installers? The reason is that the conda package manner makes it easier to install Python packages alongside CIAO, so the choice of which installer comes down to several questions:

Do you need to use other Python packages (e.g. AstroPy or SciPy) with CIAO?

If you want to install complex Python packages then you should use the conda install.

You can install Python packages into CIAO installed with ciao-install using pip, but it is a more-restrictive environment. For example, the compilers used to build CIAO with ciao-install are likely to be significantly-older than the compilers on your OS.