64-bit Ioncube vs 32-bit

If you have legacy account then you need to install 32 bit Ioncube loader because of the 32 bit server architecture. In case of PBA, 64 bit Ioncube needs to be installed. If you are installing it using easyapache on cpanel server you don't have to worry about the binary and server architectures, as cpanel takes care of correct version.

For plesk you need to choose correct version according to your server architecture. Since the installation procedure is same for both versions, You just need to download correct tar file. If you are not sure which version to install, PLease contact your hosting provider