Either one, doesn't really matter. There is also TurckMMCache which is the best of them all (if it was fully compatible with the newer versions of php and apache that is)
Zend is included in the customapache tool and can be installed by doing:
Code:
cd /usr/local/directadmin/customapache
./build zend
To install ioncube would require you to to it manually following the instructions on their website.
Most clients will use Zend encoded scripts and ioncube installation is not necassarily required since the ioncube decoder can be included in packages as far as I know.