I think it's really a matter of personal preference. More people are using rails_admin[*], but I prefer active_admin.
The reasons I chose ActiveAdmin were predominantly that it was developed with assumptions that I would make myself: using gems like devise and inherited_resources, and using a nice form builder (formtastic, though I would have chosen simple_form).
I actually prefer that not all models are exposed to the admin by default, though rails_admin supports that with 'config.included_models'.
[*] Watches/forks on https://github.com/gregbell/active_admin vs https://github.com/sferik/rails_admin - though the gap is closing.